Move contributors list to credits.md

.all-contributorsrc

@@ -4,7 +4,7 @@
   "repoType": "github",
   "repoHost": "https://github.com",
   "files": [
-    "README.md"
+    "docs/pages/pmd/projectdocs/credits.md"
   ],
   "imageSize": 100,
   "commit": true,

CONTRIBUTING.md

@@ -61,11 +61,16 @@ be imported into a fresh workspace.
 
 We use [All Contributors](https://allcontributors.org/en).
 
+To add yourself to the table of contributors, follow the
+[bot usage instructions](https://allcontributors.org/docs/en/bot/usage) ;).
+
+Or use the CLI:
+
 1. Install the CLI: `npm i` (in PMD's top level directory)
 2. Add yourself: `npx all-contributors add <username> <contribution>`
 
 Where `username` is your GitHub username and `contribution` is a `,`-separated list
 of contributions. See [Emoji Key](https://allcontributors.org/docs/en/emoji-key) for a list
-of valid types.
+of valid types. Common types are: "code", "doc", "bug", "blog", "talk", "test", "tutorial".
 
 See also [cli documentation](https://allcontributors.org/docs/en/cli/usage)