update readme for contrib packages

[brettmc]

Following on from Laravel's readme updates, apply the same to other readme files. Add some badges, and a note to make clear that people are probably looking at a read-only subtree split.
In passing, update wordpress dockerfile and metapackage to use RC

[codecov]

Codecov Report

Merging #200 (071202c) into main (039df4e) will not change coverage.
The diff coverage is n/a.

❗ Current head 071202c differs from pull request most recent head 6854c0a. Consider uploading reports for the commit 6854c0a to get more accurate results

@@            Coverage Diff            @@
##               main     #200   +/-   ##
=========================================
  Coverage     34.28%   34.28%           
  Complexity      824      824           
=========================================
  Files            73       73           
  Lines          3138     3138           
=========================================
  Hits           1076     1076           
  Misses         2062     2062           

Flag	Coverage Δ
7.4	46.34% <ø> (ø)
8.0	34.31% <ø> (ø)
8.1	34.34% <ø> (ø)
8.2	34.23% <ø> (ø)

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