Updated installation instructions for submodules

Added `--init --recursive` to installation instructions
as per @bcoca's suggestion on ansible-project mailing list
Merge pull request #9185 from willthames/submodule_install_docs

Updated installation instructions for submodules
Updated version info to include submodule information

`ansible --version` etc. now include information about submodules

```
ansible 1.8 (submodule_ansible_version ffee9a8fe0) last updated 2014/09/28 11:03:14 (GMT +1000)
  lib/ansible/modules/core: (ec2_snapshot_remove 3a77c31ecb) last updated 2014/09/27 18:23:31 (GMT +1000)
  lib/ansible/modules/extras: (detached HEAD 110250d344) last updated 2014/09/27 14:33:42 (GMT +1000)
```

Also improved handling of detached HEAD when printing out version
information.
Cater for uninitialized submodules

Output a useful message if `git submodule update --init --recursive` not
yet performed

```
$ ansible --version
ansible 1.8 (submodule_ansible_version 59ae596484) last updated 2014/09/28 13:20:51 (GMT +1000)
  lib/ansible/modules/core: (detached HEAD 617a52b20d) last updated 2014/09/28 13:15:25 (GMT +1000)
  lib/ansible/modules/extras:  not found - use git submodule update --init lib/ansible/modules/extras
```
If submodules are not found, don't error out.
Add some helpful detection if core modules are not found for those running on the development branch.
Add module search path to --version output.