Add link to Github release page for downloading stable releases.

Intentionally omitting git checkout <tag> alternative for simplicity.
Those that now how git works, will know it anyway.

README.md

@@ -21,6 +21,11 @@ HOW TO USE IT
 
 Note: For a minimal checkout, omit the `--recursive` option. This will leave out extra plugins.
 
+**Stable releases**
+
+New features and fixes are continuously merged into the master branch, which is what the above command will check out. For the latest stable release, see the [Github Releases page](https://github.com/impress/impress.js/releases).
+
+
 ### Documentation