Add primer for writing & using Operational Transforms in WebODF

# TODO
* [x] Finish examples
* [x] Add tips on how to write easy-to-transform operations
* [x] Add historic/common pitfalls
* [x] Mention logging output and how to read it
`Keeping the transform code stateless will also ensure that the transformer can be used in environments where there is no access to the document model, for example in an operation relay server.`