github/gitignore

Most files are ignores without defining the directory.
This caused trouble with several plugins.
To fix that some files should only be ignored in the root directory.

- .htaccess is only ignored in the root directory
  due to some plugins or devs that need and create the htaccess
  for security issues in subfolders
- sitemap.xml and sitemap.xml.gz is only ignored in the root directory
  due to plugins that have an equal name and are therefor ignored
- Those files are ordered alphabetically