Add README.md.
Add README.md.

Change-Id: I15241f3bf21a652a550ec5295ba105a10a253faa
Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
Reviewed-on: https://nginx-review.googlesource.com/2440
Reviewed-by: Lizan Zhou <821ab109c028f980fd658b5ceedb38e095be5bc8@google.com>

firebase/nginx,firebase/nginx,firebase/nginx,firebase/nginx