deploy.sh: document optional environment variables like EXTRA_FILES

Documentation for:
 * #68
 * #81
 * #82

resources.whatwg.org/build/README.md

@@ -18,6 +18,12 @@ For non-local deploys, it is dependent on the following environment setup:
 - The environment variable `$ENCRYPTION_LABEL` must contain the encryption label produced by the Travis encryption process.
 - The environment variable `$DEPLOY_USER` must contain the username used to SSH into the WHATWG web server.
 
+Optional environment variables:
+- `$SERVER` is the server to deploy to.
+- `$SERVER_PUBLIC_KEY` is the public key of the deploy server, in the format of `known_hosts`.
+- `$EXTRA_FILES` are extra files to copy for each build. Shell wildcards are allowed. Example: `EXTRA_FILES="*.png"`.
+- `$POST_BUILD_STEP` is an extra step to run after each build. Evaluated with the `$DIR` variable set to the build directory. Example: `POST_BUILD_STEP='python generate-stuff.py "$DIR"'`.
+
 An example `.travis.yml` file that uses this script would then be as follows:
 
 ```yaml