Update README

Updates readme to bring inline with the recommended style guide: https://github.com/alphagov/styleguides/blob/master/use-of-READMEs.md
Specialist sector is "deprecated" terminology, so we should say that explicitly. I suggest:

```
**Topic**: a named group of sub-topics. (A deprecated name for this is "specialist sector".)
**Sub-topic**: a group of content within a topic. (A deprecated name for this is "specialist sub-sector".)
```