Added instructions to update libgit2

UPDATING-LIBGIT2.md

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+### Update Fork From Upstream
+
+- Checkout https://github.com/git-up/libgit2
+- Switch to `gitup` branch
+- Pull from upstream at https://github.com/libgit2/libgit2
+- Make sure `gitup` branch current tip is tagged with the corresponding date e.g. `2015-12-02` and push tag
+- Rebased `gitup` branch on top of upstream
+
+### Test Fork
+
+- Run `./update-xcode.sh`
+- Select `libgit2_clar` project
+- Enable address sanitizer
+- Set environment variable `ASAN_OPTIONS` to `allocator_may_return_null=1`
+- Run to run all default tests
+- Run again passing `-sonline` as an argument to run online tests
+
+### Update and Test GitUp
+
+- Force push `gitup` branch to remote
+- Force update `libgit2` submodule
+- Open GitUp Xcode project
+- Select `GitUpKit (OS X)` target and run tests
+- Verify `GitUpKit (iOS)` builds
+- Commit and push