README: be more explicit in the goals and scope

Make it clearer from the get-go that we do not aim to implement
user-facing commands from the git tool.

README.md

@@ -43,9 +43,17 @@ We ask that you not open a GitHub Issue for help, only for bug reports.
 What It Can Do
 ==============
 
-`libgit2` is already very usable and is being used in production for many
-applications including the GitHub.com site, in Plastic SCM and also powering
-Microsoft's Visual Studio tools for Git.  The library provides:
+The goal of this library is to allow its users the ability to handle Git data in
+their applications from their programming language of choice, as is used in
+production for many applications including the GitHub.com site, in Plastic SCM
+and also powering Microsoft's Visual Studio tools for Git.
+
+It does not aim to replace the git tool or its user-facing commands. Some APIs
+resemble the plumbing commands as those align closely with the concepts of the
+Git system, but most commands a user would type are out of scope for this
+library to implement directly.
+
+The library provides:
 
 * SHA conversions, formatting and shortening
 * abstracted ODB backend system