Add README paragraph on versioning

Signed-off-by: Thomas Dietrich <Thomas.Dietrich@tu-ilmenau.de>

README.md

@@ -74,3 +74,17 @@ The most important setting you need to be aware of, is, that all files in collec
 - e.g. `_addons_bindings\astro\readme.md` → http://docs.openhab.org/addons/bindings/astro/readme.html
 
 Check the mentioned files for more details.
+
+## Documentation Versioning
+
+Just as openHAB is released in versions, the documentation website provides fixed versions of the documentation articles, e.g., [docs.openhab.org/2.1.0/installation/linux.html](http://docs.openhab.org/2.1.0/installation/linux.html)
+
+Please see [this issue](https://github.com/openhab/openhab-docs/issues/520#issuecomment-339741820) for all details regarding the current implementation.
+In short, the following has to be considered:
+
+- Versions like "v2.1.0" are marked by git tags.
+- Based on tags branches like "v2.1.0-patch" are created, to include later discovered changes (like fixed links).
+- The intended base folder for the version needs to be set in `_config.yml`, e.g. `baseurl: "/2.1.0"`.
+- New versions have to be added to the dropdown menu shown on the resulting website, configured in `_includes/versioning.html`.
+- The version branch has to be generated with jekyll, the resulting content goes into the version folder identical to the above set baseurl. Execute e.g. `jekyll build -d path/to/checked-out/gh-pages-branch/2.1.0/`
+- The generated static page files under, e.g., `2.1.0/` need to be committed to `gh-pages` for GitHub Pages to pick them up and include in the generated page.