Update contributing guidance to explain PR flow

Updating the contributing guidance to explain a bit more about how we use
PR's

CONTRIBUTING.md

@@ -48,6 +48,12 @@ Please include a nice description of your changes with your PR; if we have
 to read the whole diff to figure out why you're contributing in the first
 place, you're less likely to get feedback and have your change merged in.
 
+If you are working on a particular area then feel free to submit a PR that
+highlights your work in progress (and flag in the PR title that it's not
+ready to merge). This will help in getting visibility for your fix, allow
+others to comment early on the changes and also let others know that you
+are currently working on something.
+
 ## Porting Code From Other Open-Source Projects
 
 `libgit2` is licensed under the terms of the GPL v2 with a linking
@@ -57,14 +63,17 @@ The most common case is porting code from core Git.  Git is a pure GPL
 project, which means that in order to port code to this project, we need the
 explicit permission of the author.  Check the
 [`git.git-authors`](https://github.com/libgit2/libgit2/blob/development/git.git-authors)
-file for authors who have already consented; feel free to add someone if
-you've obtained their consent.
+file for authors who have already consented.
 
 Other licenses have other requirements; check the license of the library
 you're porting code *from* to see what you need to do.  As a general rule,
 MIT and BSD (3-clause) licenses are typically no problem.  Apache 2.0
 license typically doesn't work due to GPL incompatibility.
 
+If you are pulling in code from core Git, another project or code you've pulled from 
+a forum / Stack Overflow then please flag this in your PR and also make sure you've
+given proper credit to the original author in the code snippet.
+
 ## Style Guide
 
 `libgit2` is written in [ANSI C](http://en.wikipedia.org/wiki/ANSI_C)