Add docs

lib/index.js

@@ -25,9 +25,11 @@
  *   URL to hosted Git.
  *   If `repository` is nullish, the Git origin remote is detected.
  *   If the repository resolves to something npm understands as a Git host such
- *   as GitHub, GitLab, or Bitbucket, full URLs to that host (say,
- *   `https://github.com/remarkjs/remark-validate-links/readme.md#install`) can
+ *   as GitHub, GitLab, or Bitbucket, then full URLs to that host (say,
+ *   <https://github.com/remarkjs/remark-validate-links/blob/main/readme.md#install>) can
  *   also be checked.
+ *   If `repository` resolves to a branch (e.g. <https://github.com/remarkjs/remark-validate-links.git#main>) then links to other branches are ignored --- otherwise the plugin checks links to all branches as if they pointed to the current branch.
+ *   When the repository is automatically detected it checks links to all branches.
  *   If you’re not in a Git repository, you must pass `repository: false`
  *   explicitly.
  * @property {string} [root]

readme.md

@@ -164,8 +164,14 @@ URL to hosted Git (`string?` or `false`).
 If `repository` is nullish, the Git origin remote is detected.
 If the repository resolves to something [npm understands][package-repository] as
 a Git host such as GitHub, GitLab, or Bitbucket, then full URLs to that host
-(say, `https://github.com/remarkjs/remark-validate-links/readme.md#install`) can
-also be checked.
+(say,
+<https://github.com/remarkjs/remark-validate-links/blob/main/readme.md#install>)
+can also be checked.
+If `repository` resolves to a branch (e.g.
+<https://github.com/remarkjs/remark-validate-links.git#main>) then links to
+other branches are ignored --- otherwise the plugin checks links to all branches
+as if they pointed to the current branch.
+When the repository is automatically detected it checks links to all branches.
 If you’re not in a Git repository, you must pass `repository: false`
 explicitly.