Added 'undoing changes' section

README

@@ -706,6 +706,38 @@ dealing with conflicts
   Save the log message file, and exit. We now have completed our merge! See what
   the log shows.
 
+undoing changes
+---------------
+
+  When things stop working, sometimes starting over is easier than trying to
+  find and fix the error. One of the nice features of git is that you can
+  "go back" to any of your commits. For example, if you want to reset your
+  repository to the state it was in when you last committed, run:
+
+    git reset --hard HEAD
+
+  More generally, you can restore your local repository to ANY previous commit.
+  To do this, find the commit hash of the commit you want to restore (e.g. via
+  git log) and run:
+
+    git reset --hard <hash>
+
+  However, since this discards changes to all files in the repository, it's not
+  ideal if only one or two files are broken, and the rest have changes you
+  want to keep. Fortunately, git has a way to restore individual files. To
+  reset file1, run:
+
+    git checkout [<commit hash>] -- file1
+
+  (Note: the 'commit hash' argument is optional. If you omit it, git will
+  default to using the previous commit)
+
+  IMPORTANT: The ability to restore old versions can be incredibly useful,
+  but only if you make regular commits. There WILL be a time when you'll need
+  to reset your repository -- if your last working commit was five minutes
+  ago, this will be a minor setback; if it was five hours ago, you'll be much
+  less happy. Remember: commit early, commit often!
+
 gitk
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