Documentation: sync-and-gyp design document

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DOCS_PREVIEW= https://skia.org/dev/design/sync-and-gyp?cl=1736483002

Review URL: https://codereview.chromium.org/1736483002

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+sync-and-gyp
+============
+
+[`sync-and-gyp`](https://skia.googlesource.com/skia.git/+/master/bin/sync-and-gyp)
+is a Python program that wraps `gclient sync` and `gyp_skia`.
+Motivations for using it:
+
+-  Fewer steps to configure and compile Skia and Skia's dependencies.
+   This makes documentation cleaner, too.
+
+-  Written in Python, so it will work on all platforms.  Python is
+   already necessary for gyp.
+
+-  Sets up gclient better than `gclient config`, which has been broken.
+
+-  Checks to see if the `DEPS` file has changed since it last ran
+   `gclient sync`.  If not, it skips that step.
+
+-  Checks to see if gyp needs to be re-run (it checks environment
+   variables and changed or added files); if not, it skips running
+   `gyp_skia`.
+
+-  Since running `sync-and-gyp` is fast when it can do nothing, it is
+   easy to do before every recompile of Skia.  This is a good habit.