add unstable prefix section to proposal template

proposals/0000-proposal-template.md

@@ -93,3 +93,13 @@ of concerns where possible.*
 
 By having a template available, people would know what the desired detail for a proposal is. This is not
 considered a risk because it is important that people understand the proposal process from start to end.
+
+## Unstable prefix
+
+*If a proposal is implemented before it is included in the spec, then implementers must ensure that the
+implementation is compatible with the final version that lands in the spec. This generally means that
+experimental implementations should use `/unstable` endpoints, and use vendor prefixes where necessary.
+For more information, see [MSC2324](https://github.com/matrix-org/matrix-doc/pull/2324). This section
+should be used to document things such as what endpoints and names are being used while the feature is
+in development, the name of the unstable feature flag to use to detect support for the feature, or what
+migration steps are needed to switch to newer versions of the proposal.*