README: add a note about libgit2-dev from the system
@@ -26,6 +26,10 @@ All Debian-derived Linux distros provide `apt`:
$ sudo apt install libgit2-dev cmake pkg-config
```
+Note that you only need `libgit2-dev` if you want to build with the system
+libgit2 rather than the vendored version. In this case, note that the major and
+minor versions of libgit2 and rugged must match.
+
### Mac
On OS X, after installing [Homebrew](http://brew.sh/), you can get the required packages with:
```bash
