Merge branch 'issue-5-readme' into 'master'

README.md: copy-edit slightly

Closes #5

See merge request !32

README.md

 # Shared Ansible roles
 
-This repository contains DevOps group shared ansible roles.
+This repository contains DevOps group shared ansible roles. It can be paired
+with the [Ansible role fetcher
+utility](https://gitlab.developers.cam.ac.uk/uis/devops/tools/ansibleroles) to
+provide shared roles for Ansible playbooks.
 
 ## Best Practices
+
 All roles in this repository should:
+
 - Have a README
 - Have a usage example
 - Assert all required variables are set
-- Be idempotent
-- Be compose-able
-- Be orthogonal
+- Be idempotent: running the role twice should have the same effect as running
+    it once.
+- Be compose-able: roles should configure one small thing so that they may be
+    combined together to solve bigger problems. Small roles should support being
+    include-ed within larger roles.
+- Be orthogonal: roles should not replace files likely to be touched by other
+    roles.
 - Should pass ansible-lint checking
 
-
-
-
-Roles in this repo must be general purpose with all settings that can be set to a sensible default put in the defaults file.
+Roles in this repo must be general purpose with all settings that can be set to
+a sensible default put in the defaults file.
 
 ## Variables
+
 Any settings where a default makes no sense must assert the variable is set.
 
 ## READMEs
+
 Every role in here must have a `README.md` based on the `template-role-README.md`
 
 ## Check mode
+
 Roles here must be able to be run in check mode on an configured system
 
 ## Changed
 
-Roles here must be able to be run on a configured system without returning any `changed` states. 
-ie. running the playbook twice once will configure the system the second will find everything working correctly
+Roles here must be able to be run on a configured system without returning any
+`changed` states. That is to say: when running the playbook twice, the first run
+will configure the system the second will find everything working correctly and
+make no changes.
 
 ## Lint
 
-Every role in here will be automatically linted by ansible-lint and must pass before being accepted here
+Every role in here will be automatically linted by ansible-lint and must pass
+before being accepted here
 
 ## Automatic testing
 
->***TODO*** Every role here will be automatically tested on Ubuntu 16.04, 18.04 and CentOS 7 and must assert if it can't be run on one of these
+> ***TODO*** Every role here will be automatically tested on Ubuntu 16.04, 18.04
+> and CentOS 7 and must assert if it can't be run on one of these