docs: README.md: switch to https URLs (#1278)

README.md

 Documentation (incomplete at this time, contribute if you can!) may be found at
-http://gluon.readthedocs.org/.
+https://gluon.readthedocs.org/.
 
 If you're new to Gluon and ready to get your feet wet, have a look at the
-[Getting Started Guide](http://gluon.readthedocs.org/en/latest/user/getting_started.html).
+[Getting Started Guide](https://gluon.readthedocs.org/en/latest/user/getting_started.html).
 
-**Gluon IRC channel: `#gluon` in [hackint](http://hackint.org/)**
+**Gluon IRC channel: `#gluon` in [hackint](https://hackint.org/)**
 
 ## Issues & Feature requests