Update README a bit and add Contributing info

BUG=

Review-Url: https://codereview.chromium.org/2017373002

README → README.md

-This package contains tools for working with Chromium development.
-
-The "gclient" wrapper knows how to keep this repository updated to
-the latest versions of these tools as found at:
-
-    https://chromium.googlesource.com/chromium/tools/depot_tools/+/master/gclient.py
-
-This package contains:
+This package contains tools for working with Chromium development:
 
   chrome-update-create-task.bat
     Creates a scheduled task to do an automatic local chromium build every day.
@@ -25,10 +18,38 @@ This package contains:
     More info at:
     http://code.google.com/p/gclient/
 
-Note: svn and python will be installed automatically if not accessible (on
-Windows only).
+It updates itself automatically when running `gclient` tool. To disable
+auto update, set the environment variable DEPOT_TOOLS_UPDATE=0
 
-To update this distribution manually, run .\update_depot_tools.bat on Windows,
+To update package manually, run .\update_depot_tools.bat on Windows,
 or ./update_depot_tools on Linux or Mac.
 
-To disable automatic updating, set the environment variable DEPOT_TOOLS_UPDATE=0
+Note: on Windows if svn, git and python are not accessible, they will be
+downloaded too.
+
+
+## Contributing
+
+The "gclient" wrapper knows how to keep this repository updated to
+the latest versions of these tools as found at:
+
+    https://chromium.googlesource.com/chromium/tools/depot_tools.git
+
+To contribute change for review:
+
+    git new-branch <somename>
+    git add <yourchanges>
+    git commit
+    # find reviewers
+    git cl owners
+    git log <yourfiles>
+    # upload
+    git cl upload -r reviewer1@chromium.org,reviewer2 --send-mail
+    # open https://codereview.chromium.org/ and send mail
+
+    # if change is approved, flag it to be commited
+    git cl set_commit
+    # if change needs more work
+    git rebase-update
+    ...
+    git cl upload