Tracking issue: documentation

[nrc]

For initial release

• [x] Introduction (readme, include in rustdoc, check crate root docs)
• Public API of main crate
  • [x] RawClient
  • [ ] ColumnFamily
  • [x] TransactionClient
  • [ ] Snapshot
  • [x] Transaction
  • [x] Config
  • [ ] SecurityManager
  • [ ] BoundRange
  • [ ] Error
  • [ ] ErrorKind
  • [ ] Key
  • [ ] KvPair
  • [ ] Result
  • [ ] Timestamp
  • [ ] ToOwnedRange
  • [ ] Value
• [ ] examples (each example is introduced, well-commented, linked from docs)
• [ ] enumerate which parts of TiKV's API we support/don't support

For later

• [ ] Public API of all crates

[nrc]

There's still a lot of work to do here, but I believe that #269 gets us to a state where we can make a 0.1 release.