Add documentation about Enum

[Arfey]

In version2 (branch) u need to add to introduction part documentation about Enum.

For that u can use old documentation and also tests which connected with current part. As example u can use this PR.

In this task i recommend to add some example of usage and some reach description.

Please don't forget:

• your MR u need to do to the version2 (branch)
• add screenshot with your part of documentation
• write below that u want to take it to work

[holdbar]

I'd like to take this issue.

[Arfey]

👍

[holdbar]

https://github.com/Deepwalker/trafaret/pull/85