doc: update CONTRIBUTING.md

Update the procedures by using the source at the Node.js website, since they
clear some doubts on source/destination branches for contributions.

Node.js source @ http://www.nodejs.org/documentation/contributing/
Instead I prefer:
```
Fixes: https://github.com/joyent/node/issues/1234
```
The full URL is easier to work with. Using the `:` makes it more uniform with the use of `Reviewed-by:` and `Signed-off-by:`.