Docs: Fix broken links from README.md

Thanks to Alexander Kolesnik (alexkol75@gmail.com) for noticing this.

BUG=
R=fbarchard@google.com

Review URL: https://codereview.chromium.org/1805523002 .

README.md

@@ -14,5 +14,5 @@ See [Getting started] [1] for instructions on how to get started developing.
 
 You can also browse the [docs directory] [2] for more documentation.
 
-[1]: docs/getting_started.md
-[2]: docs/
+[1]: https://chromium.googlesource.com/libyuv/libyuv/+/master/docs/getting_started.md
+[2]: https://chromium.googlesource.com/libyuv/libyuv/+/master/docs/