Update README.md: Add ansible-galaxy collection command for ansible 2.9+

Add command and information about ansible-galaxy collection install. Installing collections
using the ansible-galaxy command is only supported with ansible 2.9+. The mazer tool can
be used for to install the collection for ansible 2.8:

README.md

@@ -101,6 +101,14 @@ There are RPM packages available for Fedora 29+. These are installing the roles
 
 This command will get the whole collection from galaxy:
 
+```bash
+ansible-galaxy collection install freeipa.ansible_freeipa
+```
+
+Installing collections using the ansible-galaxy command is only supported with ansible 2.9+.
+
+The mazer tool can be used for to install the collection for ansible 2.8:
+
 ```bash
 mazer install freeipa.ansible_freeipa
 ```