README: clarify the linking exception

We say that you can link libgit2 "unmodified"... "without having to release
its source code". Clarify that you can modify libgit2 - but you must
release _its_ source code back - and you can link libgit2 without having
to release _your software's_ source code.

Author: ethomson

README.md

@@ -25,10 +25,11 @@ and on Git hosting providers like [GitHub](https://github.com/),
 We perform the merge every time you click "merge pull request".
 
 `libgit2` is licensed under a **very permissive license** (GPLv2 with a special
-Linking Exception).  This basically means that you can link it (unmodified)
-with any kind of software without having to release its source code.
-Additionally, the example code has been released to the public domain (see the
-[separate license](examples/COPYING) for more information).
+Linking Exception). The GPLv2 requires you to make changes you make _to libgit2
+itself_ available publicly, but the linking exception means that you can link
+libgit2 with your software without it becoming "infected" by the libgit2
+license. Additionally, the example code has been released to the public
+domain (see the [separate license](examples/COPYING) for more information).
 
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