add to readme

[Alexey-T]

before section 'configuration' add--

Plugin can also show buttons for Code-Tree nodes. This must be enabled first
in the config file - turn on the option "code_navigation".
Clicking the buttons from the code-tree shows the same tree-popup but with
a different content - with symbols of the documents. Clicking the nodes
in the tree-popup scrolls the document to different symbols.

[halfbrained]

Thanks, updated