Created Squashing Commits (markdown)

Squashing-Commits.md

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+We try to keep the history for pdf.js relatively clean and we may ask contributors to "squash" their commits before we merge.  
+
+**Note:**
+These directions assume that you named the Mozilla pdf.js repo (not your fork) `upstream` and you have a branch called `super-feature`.
+
+1. Add the following to your git config. Either the global one or the config within your pdf.js fork (`.git/config`).
+```bash
+[alias]
+	squash = !sh -c 'git checkout upstream/master && git merge --no-commit --squash $0 && git checkout -B $0 && git commit -e'
+```
+1. Then you simply need to do:
+```bash
+git squash super-feature
+```
+1. That will bring up your editor allow you to put in the commit message you want.
+1. Then update the pull request:
+```bash
+git push --force origin super-feature
+```