Mention gmf in the README
Mention gmf in the README

And provide a link to the gmf examples.

Fixes #315.

adube/ngeo,pgiraud/ngeo,kalbermattenm/ngeo,camptocamp/ngeo,camptocamp/ngeo,pgiraud/ngeo,adube/ngeo,kalbermattenm/ngeo,ger-benjamin/ngeo,ger-benjamin/ngeo,ger-benjamin/ngeo,camptocamp/ngeo,Jenselme/ngeo,Jenselme/ngeo,adube/ngeo,Jenselme/ngeo,pgiraud/ngeo,camptocamp/ngeo,adube/ngeo,camptocamp/ngeo,kalbermattenm/ngeo,Jenselme/ngeo