Add "Document Symbols" to status feature list

This is very similar to sourcekitd's "document structure", which can provide a hierarchical view of entities in the current file.

README.md

@@ -89,6 +89,7 @@ SourceKit-LSP is still in early development, so you may run into rough edges wit
 | Formatting | ❌ | |
 | Folding | ❌ | |
 | Syntax Highlighting | ❌ | Not currently part of LSP. |
+| Document Symbols | ❌ |  |
 
 
 ### Caveats