add lsp-workspace-symbol-search example in README.md

Author: prabirshrestha

README.md

@@ -31,6 +31,7 @@ function! s:on_lsp_buffer_enabled() abort
     if exists('+tagfunc') | setlocal tagfunc=lsp#tagfunc | endif
     nmap <buffer> gd <plug>(lsp-definition)
     nmap <buffer> gs <plug>(lsp-document-symbol-search)
+    nmap <buffer> gS <plug>(lsp-workspace-symbol-search)
     nmap <buffer> gr <plug>(lsp-references)
     nmap <buffer> gi <plug>(lsp-implementation)
     nmap <buffer> gt <plug>(lsp-type-definition)