Add a code of conduct.

It's verbatim from https://www.contributor-covenant.org/, except for
adding in the contact info for reports.

Fixes #20.

Signed-off-by: David Anderson <dave@natulte.net>

CODE_OF_CONDUCT.md

@@ -0,0 +1,135 @@
+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to make participation
+in our community a harassment-free experience for everyone, regardless
+of age, body size, visible or invisible disability, ethnicity, sex
+characteristics, gender identity and expression, level of experience,
+education, socio-economic status, nationality, personal appearance,
+race, religion, or sexual identity and orientation.
+
+We pledge to act and interact in ways that contribute to an open,
+welcoming, diverse, inclusive, and healthy community.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment for
+our community include:
+
+* Demonstrating empathy and kindness toward other people
+* Being respectful of differing opinions, viewpoints, and experiences
+* Giving and gracefully accepting constructive feedback
+* Accepting responsibility and apologizing to those affected by our
+  mistakes, and learning from the experience
+* Focusing on what is best not just for us as individuals, but for the
+  overall community
+
+Examples of unacceptable behavior include:
+
+* The use of sexualized language or imagery, and sexual attention or
+  advances of any kind
+* Trolling, insulting or derogatory comments, and personal or
+  political attacks
+* Public or private harassment
+* Publishing others' private information, such as a physical or email
+  address, without their explicit permission
+* Other conduct which could reasonably be considered inappropriate in
+  a professional setting
+
+## Enforcement Responsibilities
+
+Community leaders are responsible for clarifying and enforcing our
+standards of acceptable behavior and will take appropriate and fair
+corrective action in response to any behavior that they deem
+inappropriate, threatening, offensive, or harmful.
+
+Community leaders have the right and responsibility to remove, edit,
+or reject comments, commits, code, wiki edits, issues, and other
+contributions that are not aligned to this Code of Conduct, and will
+communicate reasons for moderation decisions when appropriate.
+
+## Scope
+
+This Code of Conduct applies within all community spaces, and also
+applies when an individual is officially representing the community in
+public spaces. Examples of representing our community include using an
+official e-mail address, posting via an official social media account,
+or acting as an appointed representative at an online or offline
+event.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior
+may be reported to the community leaders responsible for enforcement
+at [info@tailscale.com](mailto:info@tailscale.com). All complaints
+will be reviewed and investigated promptly and fairly.
+
+All community leaders are obligated to respect the privacy and
+security of the reporter of any incident.
+
+## Enforcement Guidelines
+
+Community leaders will follow these Community Impact Guidelines in
+determining the consequences for any action they deem in violation of
+this Code of Conduct:
+
+### 1. Correction
+
+**Community Impact**: Use of inappropriate language or other behavior
+deemed unprofessional or unwelcome in the community.
+
+**Consequence**: A private, written warning from community leaders,
+providing clarity around the nature of the violation and an
+explanation of why the behavior was inappropriate. A public apology
+may be requested.
+
+### 2. Warning
+
+**Community Impact**: A violation through a single incident or series
+of actions.
+
+**Consequence**: A warning with consequences for continued
+behavior. No interaction with the people involved, including
+unsolicited interaction with those enforcing the Code of Conduct, for
+a specified period of time. This includes avoiding interactions in
+community spaces as well as external channels like social
+media. Violating these terms may lead to a temporary or permanent ban.
+
+### 3. Temporary Ban
+
+**Community Impact**: A serious violation of community standards,
+including sustained inappropriate behavior.
+
+**Consequence**: A temporary ban from any sort of interaction or
+public communication with the community for a specified period of
+time. No public or private interaction with the people involved,
+including unsolicited interaction with those enforcing the Code of
+Conduct, is allowed during this period. Violating these terms may lead
+to a permanent ban.
+
+### 4. Permanent Ban
+
+**Community Impact**: Demonstrating a pattern of violation of
+community standards, including sustained inappropriate behavior,
+harassment of an individual, or aggression toward or disparagement of
+classes of individuals.
+
+**Consequence**: A permanent ban from any sort of public interaction
+within the community.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor
+Covenant][homepage], version 2.0, available at
+https://www.contributor-covenant.org/version/2/0/code_of_conduct.html.
+
+Community Impact Guidelines were inspired by [Mozilla's code of
+conduct enforcement ladder](https://github.com/mozilla/diversity).
+
+[homepage]: https://www.contributor-covenant.org
+
+For answers to common questions about this code of conduct, see the
+FAQ at https://www.contributor-covenant.org/faq. Translations are
+available at https://www.contributor-covenant.org/translations.
+