Fix collection install (#9)

Without this, I cannot get it to install `main`. Moving forwards, we
will probably want to specify `vx.y.z`, but for now let's fix this.

README.md

@@ -38,6 +38,7 @@ You can also include it in a `requirements.yml` file and install it via
 collections:
   - name: mirsg.xnat
     source: https://github.com/UCL-MIRSG/ansible-collection-xnat.git
+    type: git
     version: main
 ```