Added a guide about managing software dependencies

This follows on from a conversation on the GOV.UK developers mailing list.
Minor tweaks

- Style guide
- Spelling
minor typos
Fix typos
as per feedback, make case for specific version ot just general
as per feedback, strengthening guidance on use of lists
as per feedback, adding useful cross-links
correct for minor style preference
fix typo
Merge pull request #362 from alphagov/dependency-management

Added a guide about managing software dependencies