Add noun after mentions of Open edX and change email address for requesting entity agreement.

The openedx.org/cla link is a redirection to Docusign.  If the form has
to change in the future, we can update the redirect at openedx.org.

There are other changes to make to CONTRIBUTING.rst, but those will have
to wait.

The openedx.org/cla link is a redirection to Docusign.  If the form has
to change in the future, we can update the redirect at openedx.org.

There are other changes to make to CONTRIBUTING.rst, but those will have
to wait.

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.