docs(README): add neotest-phpunit adapter (#65)

README.md

@@ -81,6 +81,7 @@ To get started you will also need to install an adapter for your test runner.
 | rspec           |     [neotest-rspec](https://github.com/olimorris/neotest-rspec)      |
 | dart, flutter   |       [neotest-dart](https://github.com/sidlatau/neotest-dart)       |
 | testthat        | [neotest-testthat](https://github.com/shunsambongi/neotest-testthat) |
+| phpunit         | [neotest-phpunit](https://github.com/olimorris/neotest-phpunit)      |
 
 For any runner without an adapter you can use [neotest-vim-test](https://github.com/nvim-neotest/neotest-vim-test) which supports any runner that vim-test supports.
 The vim-test adapter does not support some of the more advanced features such as error locations or per-test output.