Be more specific about HEAD^ vs HEAD~ (k88hudson#122)

* Be more specific about HEAD^ vs HEAD~

* add more

* revise again

Author: Tatsujinichi

README.md

@@ -383,7 +383,13 @@ Reset the branch master to the previous commit:
 (master)$ git reset --hard HEAD^
 ```
 
-`HEAD^` is short for `HEAD^1`. You can reset further through the generations by specifying which `HEAD` to set to.
+`HEAD^` is short for `HEAD^1`. This stands for the first parent of `HEAD`, similarly `HEAD^2` stands for the
+
+second parent of the commit (merges can have 2 parents).
+
+Note that `HEAD^2` is **not** the same as `HEAD~2`
+
+(see [this link](http://www.paulboxley.com/blog/2011/06/git-caret-and-tilde) for more information).
 
 Alternatively, if you don't want to use `HEAD^`, find out what the commit hash you want to set your master branch to (`git log` should do the trick). Then reset to that hash. `git push` will make sure that this change is reflected on your remote.