Add notes about css compilation for Elastic skin

INSTALL

@@ -60,7 +60,12 @@ INSTALLATION
 5. Point your browser to http://url-to-roundcube/installer/
 6. Follow the instructions of the install script (or see MANUAL CONFIGURATION)
 7. After creating and testing the configuration, remove the installer directory
-8. Check Known Issues section of this file
+8. If you use git sources compile css files for the Elastic skin:
+   $ cd skins/elastic
+   $ lessc -x styles/styles.less > styles/styles.css
+   $ lessc -x styles/print.less > styles/print.css
+   $ lessc -x styles/embed.less > styles/embed.css
+9. Check Known Issues section of this file
 
 
 CONFIGURATION HINTS

UPGRADING

@@ -53,6 +53,11 @@ it on a unix system, you need to do the following operations by hand:
    - run `php composer.phar install --no-dev`.
 4c. If you use git sources or the release package without dependencies
    update javascript dependencies by executing `bin/install-jsdeps.sh` script.
+4d. If you use git sources compile css files for the Elastic skin:
+   $ cd skins/elastic
+   $ lessc -x styles/styles.less > styles/styles.css
+   $ lessc -x styles/print.less > styles/print.css
+   $ lessc -x styles/embed.less > styles/embed.css
 5. Run `./bin/update.sh` from the commandline OR
    open http://url-to-roundcube/installer/ in a browser and choose "3 Test config".
    To enable the latter one, you have to temporary set 'enable_installer'