Add note about exclude path interpretation

See anishathalye#292.

README.md

@@ -201,6 +201,8 @@ When `glob: True`, Dotbot uses [glob.glob](https://docs.python.org/3/library/glo
 
 However, due to the design of `glob.glob`, using a glob pattern such as `config/*`, will **not** match items that begin with `.`. To specifically capture items that being with `.`, you will need to include the `.` in the pattern, like this: `config/.*`.
 
+When using glob with the `exclude:` option, the paths in the exclude paths should be relative to the base directory, same as the glob pattern itself. For example, if a glob pattern `vim/*` matches directories `vim/autoload`, `vim/ftdetect`, `vim/ftplugin`, and `vim/spell`, and you want to ignore the spell directory, then you should use `exclude: ["vim/spell"]` (not just `"spell"`).
+
 #### Example
 
 ```yaml