Add docs for building the docs locally

docs/README.md

+To build the docs locally, you need to have docker installed.  The docs are built using a custom [docker](https://www.docker.com/)
+image and [mkdocs](http://www.mkdocs.org/).
+
+Build the `grafana/docs-base:latest` image:
+
+```
+$ git clone https://github.com/grafana/docs-base
+$ cd docs-base
+$ make docs-build
+```
+
+To build the docs:
+```
+$ cd docs
+$ make docs
+```
+
+Open [localhost:8100](http://localhost:8180) to view the docs.