Add apache requirements to README

[cortho]

It took me quite a while finding out, that both mod_rewrite and mod_headers are required to have oauth2 run on apache.

For me is is not sufficient, that this information might be found somewhere in the server docs (see #49 (comment)).

[SamuAlfageme]

@cortho indeed; getting this info at first sight both on the app's README and its marketplace page will help more admins getting it right without wasting time. Thanks for the tip!

Also ref. this issue for discussion: owncloud/core#30042.

[settermjd]

Are any specific modules required for NGINX?

[settermjd]

While NGINX isn't officially supported, as it has a lot of community interest and usage, it'd be handy to know and to document.

[settermjd]

I've added a note about mod_headers being mandatory for OAuth2 access in installation/source_installation.html, in the admin manual in recent changes in #3464.