Add docs building instructions.

docs/README.md

@@ -1 +1,21 @@
 # Grafana-Zabbix Documentation
+
+## Building docs
+To build this docs on your computer you need [git-lfs](https://git-lfs.github.com/) and [mkdocs](http://www.mkdocs.org/) installed.
+
+Clone repo
+```
+git clone https://github.com/alexanderzobnin/grafana-zabbix
+```
+Check images in `docs/sources/img/`. If this folder is empty, run 
+```
+git lfs fetch --all
+```
+
+Build docs
+```
+cd docs/
+mkdocs build --clean
+```
+
+Built docs will be placed in `site/` directory.