Update CONTRIBUTING.md

Add a quick note, based on https://github.com/hotosm/learnosm/issues/225 's conversation - the quickest way to propose minor changes is via the online editing tools; running the full site or translations are more advanced topics.
Add links and a quick start for minor edits
Fix #195 by updating the relevant screenshot
Reduce to 512px wide
Merge pull request #236 from Nick-Tallguy/gh-pages

IRC section added & small grammar & punctuation amendments
Merge pull request #237 from CloCkWeRX/patch-9

Update CONTRIBUTING.md
Merge pull request #238 from CloCkWeRX/patch-10

Add links and a quick start for minor edits
Merge pull request #240 from CloCkWeRX/fix_195

Fix #195 by updating the relevant screenshot
Additional info on using IRC and slight amendment to general advice.