doc: add docs for env vars

README.md

@@ -11,10 +11,12 @@ It works on browsers with [WebExtensions](https://developer.mozilla.org/en-US/Ad
 
 More details can be found [here](https://violentmonkey.github.io/).
 
-## Related projects
+## Environment Variables
 
-- [Violentmonkey for Opera Presto](https://github.com/violentmonkey/violentmonkey-oex)
-- [Violentmonkey for Maxthon](https://github.com/violentmonkey/violentmonkey-mx)
+The following environment variables will be injected at compilation time for some features to work:
+
+- `SYNC_GOOGLE_CLIENT_ID` / `SYNC_GOOGLE_CLIENT_SECRET` - Google sync service
+- `SYNC_ONEDRIVE_CLIENT_ID` / `SYNC_ONEDRIVE_CLIENT_SECRET` - OneDrive sync service
 
 ## Workflows
 
@@ -57,3 +59,8 @@ $ yarn bump
 # Create a patch release
 $ yarn version --patch
 ```
+
+## Related Projects
+
+- [Violentmonkey for Opera Presto](https://github.com/violentmonkey/violentmonkey-oex)
+- [Violentmonkey for Maxthon](https://github.com/violentmonkey/violentmonkey-mx)