add some details to the README.md and copy it to the repo description on Docker Hub

Author: Paul van Leeuwen

README.md

@@ -1,2 +1,13 @@
 # docker-php-composer
-Adds composer to standard php images.
+
+Adds composer to these standard php images: https://hub.docker.com/_/php/
+
+To be more exact:
+* installs git (used by composer)
+* copies composer.phar to /usr/local/bin/composer and makes it executable
+
+Because I install git via apt-get and because the Alpine images need a different way to do this, I excluded them for now. So far I did not miss them yet. If you have a need for those, feel free to contact me (pjvleeuwen) at gmail (@gmail.com).
+
+Sources: https://github.com/pjvleeuwen/docker-php-composer/
+Images: https://hub.docker.com/r/pjvleeuwen/php-composer/
+