Add user document about feature of File-level routing

[amyangfei]

Feature Request

Describe your feature request related problem:

This PR https://github.com/pingcap/tidb-lightning/pull/366 supports file level routing in lightning, but we lack the user document for it now.

Describe the feature you'd like:

• [ ] Add document about this feature. ref Lightning RFC 5: File-Level Routing
• [ ] Make Lightning RFCs public

Describe alternatives you've considered:

• The schema-name or table-name in file level routing is case sensitive, should we add case insensitive support?