Add initial draft of installation guide

Still incomplete.
Add initial draft of client usage guide

Still incomplete.
Merge branch 'master' into peng/initial-documentation
Minor documentation fixups and additions
Merge branch 'master' into peng/initial-documentation
Update README.md and CONTRIBUTING.md
Add initial draft of accounts usage guide
Add initial draft of usage overview guide
Merge pull request #29 from 3dna/peng/initial-documentation

Add initial documentation

Add initial versions of all the guides.