migrate GN docs from the wiki to the repo.

tools/gn/docs/hacking.md

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+# Hacking on the GN binary itself
+
+## Building GN itself
+
+GN is part of the Chromium tree. If you have a Chromium checkout, you
+already have the source and you can do `ninja -C out/Debug gn` to
+build it.
+
+To build gn using gn, run (in the root `src` directory):
+
+```
+gn gen out/Default
+ninja -C out/Default gn
+```
+
+Change `out/Default` as necessary to put the build directory where you
+want.
+
+## Running GN's unit tests
+
+```
+ninja -C out/Default gn_unittests && out/Default/gn_unittests
+```