Ignore composer files in certain directories.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | ---
| Refs tickets  | ---
| License       | MIT
| Doc PR        | ---

e.g. the Extension library [has a composer.json example file in the docs directory](https://github.com/craigh/ExtensionLibrary/blob/master/Zikula/Module/ExtensionLibraryModule/docs/composer.json).