Slack should no longer be used for asking for help. Instead, people
should head to the discussion forums at https://discuss.openedx.com.

Because no one actually uses that page

I was going to make a more substantial change, but then realized why I
shouldn't.  Along the way, I saw these three words that should be
corrected.  So I was left with three very minor spelling fixes.  Should
I abandon them?  Seemed unfortunate.  So I'm committing them.

These are the kinds of decisions software engineers have to make....

Thanks to @adampalay, we now have a list of byte-sized tasks, which are
useful for new contributors who want a gentle introduction to the
platform. This PR adds a link to the tasks tagged as byte-sized in the
public tracker.

Currently the URL points to a wrong URL, fixed it by adding 'http' to the URL.

Based on the How To Contribute wiki page (and switched to markdown so
it's a simple copy-paste).

Created a CONTRIBUTING.rst which GitHub will pick up and included a
link to the How To Contribute wiki page there. Also added the same link
to the README in the "How To Contribute" section.