Issue #540: Add downloads badge, organize all badges

[zacbaum]

Description

Organized badges on README.md

Fixes #540

Type of change

What types of changes does your code introduce to DeepReg? Put an x in the boxes that
apply

• Bugfix (non-breaking change which fixes an issue)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation Update (fix or improvement on the documentation)
• New feature (non-breaking change which adds functionality)
• Other (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR.
If you're unsure about any of them, don't hesitate to ask. We're here to help! This is
simply a reminder of what we are going to look for before merging your code.

• I have
  installed pre-commit
  using pre-commit install and formatted all changed files. If you are not
  certain, run pre-commit run --all-files.
• My commits' message styles matches
  our requested structure,
  e.g. Issue #<issue number>: detailed message.
• I have updated the
  change log file
  regarding my changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[zacbaum]

@mathpluscode @NMontanaBrown, please approve if you like the new look!

https://github.com/DeepRegNet/DeepReg/blob/540-badges/README.md

[mathpluscode]

@mathpluscode @NMontanaBrown, please approve if you like the new look!

https://github.com/DeepRegNet/DeepReg/blob/540-badges/README.md

Marvellous!!!

[codecov]

Codecov Report

Merging #541 (3d907f3) into main (e6b4f18) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #541   +/-   ##
=======================================
  Coverage   98.83%   98.83%           
=======================================
  Files          30       30           
  Lines        1894     1894           
=======================================
  Hits         1872     1872           
  Misses         22       22           

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