migrate GN docs from the wiki to the repo.

tools/gn/docs/update_binaries.md

+# How to update the [GN binaries](gn.md) that Chromium uses.
+
+## Prerequisites
+
+You'll need a checkout of Chromium, and commit access to the
+`buildtools/` repo. Check with scottmg, thakis, brettw, dpranke, or a
+member of chrome-infra to get access to that repo.
+
+## Instructions
+
+_Hopefully there will be a script that does this all for you shortly._
+
+  1. cd to your Chromium checkout.
+  2. Create a whitespace change in DEPS and upload it to create a "dummy" CL
+  3. Run the following commands:
+    1. git-cl try -b linux\_chromium\_gn\_upload\_x86
+       -b linux\_chromium\_gn\_upload\_x64
+       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
+    2. git-cl try -b mac\_chromium\_gn\_upload
+       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
+    3. git-cl try -b win8\_chromium\_gn\_upload
+       -r $GIT\_REVISION\_YOU\_WANT\_TO\_BUILD
+  4. Wait for the try jobs to finish.
+  5. If they all ran successfully, copy the digests from the tryjob build
+     log output into `src/buildtools/{mac,linux32,linux64}gn.sha1` and
+     `src/buildtools/win/gn.exe.sha1` as appropriate.
+  6. Upload a buildtools CL with the updated digests and get it reviewed
+     and committed. Make sure you note the revision of GN that you built
+     against in the commit message.
+  7. Go back to your "dummy" CL w/ the change to the DEPS file in src/
+     and update the buildtools revision to your newly-committed
+     buildtools change.
+  8. Get that reviewed and landed in Chromium. Make sure you note the
+     revision of GN that you built against in the commit message, along
+     with the revision of buildtools that you're rolling to.