Include clib's package reference.

This PR introduces a new top-level file, `package.json`, which enables this repository compatibility with [`clib`](https://github.com/clibs/clib), an open source C package manager.  By doing this, users of `clib` can quickly include the `libgit2` library within their project.

package.json

+{
+  "name": "libgit2",
+  "version": "0.27.0",
+  "repo": "https://github.com/libgit2/libgit2",
+  "description": " A cross-platform, linkable library implementation of Git that you can use in your application.",
+  "install": "mkdir build && cd build && cmake .. && cmake --build ."
+}