docs/README: document how to build locally

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

docs/README.md

@@ -0,0 +1,18 @@
+
+#### etcd Documentation
+
+This is an official etcd documentation https://etcd.readthedocs.io/en/latest.
+
+Working in progress...
+
+#### Install and build
+
+```bash
+# https://docs.readthedocs.io/en/latest/getting_started.html
+pip install sphinx sphinx-autobuild
+pip install sphinx_rtd_theme
+
+make html
+```
+
+And open `_build/html/index.html` for preview.