Document how to get tagbar working with Rust
**Is your feature request related to a problem? Please describe.**
When opening a Rust project, then a .rs file, the Tagbar is empty.

**Describe the solution you'd like**
Explain how to fill Tagbar

**Describe alternatives you've considered**
`uctags -R` in the project root to provide a tags file doesn't change anything (well, probably not relevant as other languages don't need that step to fill the tagbar)

**Additional context**
A configuration for that exists: https://github.com/rust-lang/rust.vim/blob/master/ftplugin/rust/tagbar.vim
