Add Contributing to Skia section of docs

site/dev/contrib/patch.md

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+Applying patches
+================
+
+If you are a Skia committer and have been asked to commit an
+externally-submitted patch, this is how to do it.  (This technique is useful in
+other situations too, like if you just want to try out somebody else's patch
+locally.)
+
+Notes: 
+  * For the examples below, we will assume that this is the change you want
+    to patch into your local checkout: https://codereview.appspot.com/6201055/ 
+  * These instructions should work on Mac or Linux; Windows is trickier, 
+    because there is no standard Windows "patch" tool.  
+
+See also:
+http://dev.chromium.org/developers/contributing-code#TOC-Instructions-for-Reviewer:-Checking-in-the-patch-for-a-non-committer
+
+If you use git cl, then you should be able to use the shortcut:
+
+~~~~ 
+git cl patch 6201055 
+~~~~
+
+If you use gcl, or the above doesn't work, the following should always work.
+
+1. Prepare your local workspace to accept the patch.
+
+    * cd into the root directory (usually trunk/) of the workspace where you
+      want to apply the patch.  
+    * Make sure that the workspace is up-to-date and clean (or "updated and 
+      clean enough" for your purposes).  If the codereview patch was against 
+      an old revision of the repo, you may need to sync your local workspace 
+      to that same revision...
+
+2. Download the raw patch set.
+
+    * Open the codereview web page and look for the "Download raw patch set"
+      link near the upper right-hand corner.  Right-click on that link and copy
+      it to the clipboard.  (In my case, the link is
+      https://codereview.appspot.com/download/issue6201055_1.diff ) 
+    * If you are on Linux or Mac and have "curl" or "wget" installed, you can 
+      download the patch from the command line:
+
+    ~~~~ 
+    curl https://codereview.appspot.com/download/issue6201055_1.diff
+    --output patch.txt
+    # or...
+    wget https://codereview.appspot.com/download/issue6201055_1.diff
+    --output-document=patch.txt 
+    ~~~~
+
+    * Otherwise, figure out some other way to download this file and save it as
+      'patch.txt'
+
+3. Apply this patch to your local checkout.
+
+    * You should still be in the root directory of the workspace where you want
+      to apply the patch.
+
+    ~~~~ 
+    patch -p1 <patch.txt 
+    ~~~~
+
+    * Then you can run diff and visually check the local changes.
+
+4. Complications: If the patch fails to apply, the following may be happening:
+
+   Wrong revision.  Maybe your local workspace is not up to date?  Or maybe the
+   patch was made against an old revision of the repository, and cannot be applied
+   to the latest revision?  (In that case, revert any changes and sync your
+   workspace to an older revision, then re-apply the patch.)
+