Port Update Angle from Trac

#64

Port the Update ANGLE guide from Trac.

Author: stwrt

docs/Infrastructure/UpdatingAngle.md

@@ -0,0 +1,15 @@
+# Update ANGLE
+
+## Fixing ANGLE Bugs
+
+Before you commit a WebKit patch that modifies ANGLE, please run `Tools/Scripts/update-angle --regenerate-changes`.
+This will update `Source/ThirdParty/ANGLE/changes.diff` so that people can see the diff from upstream at a glance.
+
+When fixing bugs in ANGLE, please create a new bug on [ANGLE's bug tracker](https://bugs.chromium.org/p/angleproject/issues/list) and attach the patch applied to WebKit
+so that changes can eventually be merged upstream instead of maintained locally.
+
+## Merging ANGLE from Upstream
+
+To pull in a new revision of ANGLE, run the script `Tools/Scripts/update-angle` and follow its instructions. 
+This script will attempt to update to a new version of ANGLE without losing WebKit's local changes by performing a git rebase. 
+It also helps to update the CMake build files and `ANGLE.plist`.