Change readme.md to README.md in .gitattributes
Change readme.md to README.md in .gitattributes

`.gitattributes` is case sensitive, so `readme.md` does not work, the file is still included with a `composer install`.
mhlavac/flysystem,nao-pon/flysystem,mhlavac/flysystem,twistor/flysystem,hannesvdvreken/flysystem,twistor/flysystem