Update README.md

[TomDonoghue]

Thanks for contributing. If this is your first time,
make sure to read the contributing guide.

PR Description

As I was looking through the repository related to the JOSS review, there were a couple small README comments that seemed easier to suggest in a PR than a list of suggestions.

Please feel free to edit and updates things from this initial suggestion, the main points I wanted to address are:

• updating some of the phrasing that felt a bit awkward
• noting more explicitly in the install that there is a stable release available through pip (since it only mentions dev)
• fixing a dead link for the contributing guidelines

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #84 (840cb23) into main (2ddc063) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #84   +/-   ##
=======================================
  Coverage   90.63%   90.63%           
=======================================
  Files          23       23           
  Lines         897      897           
=======================================
  Hits          813      813           
  Misses         84       84           

Help us with your feedback. Take ten seconds to tell us how you rate us.