Update release procedure (#943)
Update release procedure (#943)

Major update to release procedure here is the change in the order
of steps - PyPi and Conda publishing should now be done before
pushing to master. This is so that any problems with the publishing
steps can be resolved before packaging the release on GitHub.

This also adds some missing items from previous experience with 
the release procedure.

Some of the wording and formatting also updated in the document 
for clarity.

Part of #940

stellargraph/stellargraph,stellargraph/stellargraph