Update Galaxy meta and readme

Readme mostly formatting changes. Meta file now includes supported Ubuntu releases.

meta/main.yml

@@ -27,7 +27,7 @@ galaxy_info:
   # this branch. If Travis integration is configured, only notifications for this
   # branch will be accepted. Otherwise, in all cases, the repo's default branch
   # (usually master) will be used.
-  #github_branch:
+  github_branch: master
 
   #
   # platforms is a list of platforms, and each platform has a name and a list of versions.
@@ -39,6 +39,10 @@ galaxy_info:
   - name: Debian
     versions:
     - stretch
+  - name: Ubuntu
+    versions:
+    - Bionic
+    - Cosmic
   - name: Fedora
     versions:
     - 28