Add docs for language server support

docs/language_server.md

@@ -0,0 +1,60 @@
+# Using Psalm's Language Server
+
+Psalm now has built-in Language Server Compatibility support.
+
+It currently only supports diagnostics, with more functionality coming soon.
+
+It works well in a variety of editors:
+
+## emacs
+
+I got it working with [eglot](https://github.com/joaotavora/eglot)
+
+This is the config I used:
+
+```
+(require 'eglot)
+(add-to-list 'eglot-server-programs '(php-mode . ("php" "vendor/bin/psalm-language-server")))
+```
+
+## PhpStorm
+
+Haven't managed to get it working yet in PhpStorm. But soon.
+
+## Sublime Test
+
+I use the excellent Sublime [LSP plugin](https://github.com/tomv564/LSP) with the following config:
+
+```json
+        "psalm":
+        {
+            "command": ["php", "vendor/bin/psalm-language-server"],
+            "scopes": ["source.php", "embedding.php"],
+            "syntaxes": ["Packages/PHP/PHP.sublime-syntax"],
+            "languageId": "php"
+        }
+```
+
+## vim/neovim
+
+### ALE
+
+[Coming soon](https://github.com/w0rp/ale/pull/2008), but you can use the existing ALE Psalm plugin for now.
+
+### vim-lsp
+
+I got it working with [vim-lsp](https://github.com/prabirshrestha/vim-lsp)
+
+This is the config I used:
+
+```
+au User lsp_setup call lsp#register_server({
+     \ 'name': 'psalm-language-server',
+     \ 'cmd': {server_info->[expand('vendor/bin/psalm-language-server')]},
+     \ 'whitelist': ['php'],
+     \ })
+```
+
+## VS Code
+
+Plugin under development here: https://github.com/psalm/psalm-vscode-plugin