docs: document the config input parameter (#97)

* docs: document the config input parameter

The config input maps to codespell's --config flag in entrypoint.sh
but was missing from the README. Add a parameter section for it,
consistent with the other documented inputs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs: clarify config input vs. auto-discovered config files

setup.cfg/.codespellrc/pyproject.toml are read automatically; the
config input is for pointing at a config file in a non-standard
location. Use a non-standard path in the example to reflect that.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: ehennestad

README.md

@@ -46,6 +46,18 @@ with:
   exclude_file: src/foo
 ```
 
+### Parameter: config
+
+Path to a `codespell` config file. The file uses INI format with a `[codespell]` section.
+
+This parameter is optional. By default `codespell` automatically reads configuration from `pyproject.toml`, `setup.cfg`, and `.codespellrc` in the working directory; use this parameter only to point at a config file in a non-standard location.
+
+```yml
+uses: codespell-project/actions-codespell@v2
+with:
+  config: path/to/codespell.cfg
+```
+
 ### Parameter: skip
 
 Comma-separated list of files to skip (it accepts globs as well).