Add README.md to root directory

Add README.md to root directory

This markdown file will automatically be rendered at the lower part of
the page when users browse to
https://chromium.googlesource.com/external/webrtc

BUG=
R=tommi@webrtc.org

Committed: https://chromium.googlesource.com/external/webrtc/+/c68700f717cf582b3af19f7a48ccf989a9b56d3d

[kjellander_webrtc, 2015-09-11 06:09:20]

kjellander@webrtc.org changed reviewers:
+ niklas.enbom@webrtc.org, tommi@webrtc.org

[kjellander_webrtc, 2015-09-11 06:09:21]

Several other projects have this, which makes them show better info at the Git
URL:
https://chromium.googlesource.com/chromium/src/tools/gn
https://chromium.googlesource.com/external/gyp/
https://chromium.googlesource.com/v8/v8/

https://codereview.webrtc.org/1327273002/diff/1/README.md
File README.md (right):

https://codereview.webrtc.org/1327273002/diff/1/README.md#newcode12
README.md:12: ### Development
The 3 paragrahs above is an exact copy of the public web page. Should we have
that or just have a single line pointing at webrtc.org instead?

[tommi, 2015-09-11 07:43:32]

lgtm as is.

[kjellander_webrtc, 2015-09-11 09:04:15]

Committed patchset #1 (id:1) manually as
c68700f717cf582b3af19f7a48ccf989a9b56d3d (presubmit successful).

[commit-bot: I haz the power, 2015-09-11 09:04:16]

Patchset 1 (id:??) landed as
https://crrev.com/c68700f717cf582b3af19f7a48ccf989a9b56d3d
Cr-Commit-Position: refs/heads/master@{#9924}

[niklas.enbom, 2015-09-11 14:41:50]

lgtm
On 2015/09/11 09:04:16, commit-bot: I haz the power wrote:
> Patchset 1 (id:??) landed as
> https://crrev.com/c68700f717cf582b3af19f7a48ccf989a9b56d3d
> Cr-Commit-Position: refs/heads/master@{#9924}