JOSS Minor Changes

[aneeshnaik]

The aim of this PR is to address the 'minor' changes that the reviewers have suggested in the JOSS review process. The more substantial queries will be addressed separately.

Suggestions from @vankesteren:

• [x] Community guidelines (resolve #6)
• [x] Add scipy.stats.norm import to quickstart example (resolve #7)
• [x] Add basic example to paper (resolve #9)

Suggestions from @matt-graham (these collectively resolve #10)

• [x] Define PDF acronym on first usage
• [x] Fix "PDF function" redundancy
• [x] Citations for numpy and scipy
• [x] Fix MCMC capitalisation
• [x] Fix SGMCMCJax bibtex entry

[michael-petersen]

Part of what 'PDF function' was meant to describe was the user providing a function (in the Python sense) that computes the PDF itself. So I'm thinking it makes sense to say 'function describing the PDF', e.g. 'the user provides a Python function defining the target PDF'. Is 'Python function' over definition, or helpful to clarify the multiple uses of function?

I've followed this strategy already; but we can change if desired (hence the box left un-ticked).

[michael-petersen]

I've drafted the example; it might benefit from a bit of wordsmithing. After that, I think the checklist is done!