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

Review URL: https://codereview.webrtc.org/1327273002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9924}

ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc