add efm example to docs

notes.md

@@ -1,4 +1,4 @@
-# Override used Nix packages
+## Override used Nix packages
 
 In case you need to specify custom nix packages, you can override the default ones by specifying `on_new_config` callback with `in_shell` helper function:
 
@@ -16,7 +16,40 @@ require("lazy-lsp").setup {
 }
 ```
 
-# Install via Nix/Home Manager
+## Additional Linters and Formatters (EFM)
+
+You can use [efm-langserver](https://github.com/mattn/efm-langserver) to set up additional formatters and linters when LSP implementation is not available.
+
+For example, use following config to get [shellcheck](https://www.shellcheck.net/) diagnostic messages when writing shell scripts:
+
+```lua
+require("lazy-lsp").setup {
+  configs = {
+    efm = {
+      filetypes = { "sh" },
+      settings = {
+        languages = {
+          sh = {
+            {
+              lintCommand = "shellcheck -f gcc -x",
+              lintSource = "shellcheck",
+              lintFormats = { "%f:%l:%c: %trror: %m", "%f:%l:%c: %tarning: %m",
+                "%f:%l:%c: %tote: %m" },
+            },
+          },
+        },
+      },
+      on_new_config = function(new_config)
+        -- Override used packages to add additional dependencies
+        local nix_pkgs = { "efm-langserver", "shellcheck" }
+        new_config.cmd = require("lazy-lsp").in_shell(nix_pkgs, new_config.cmd)
+      end,
+    },
+  },
+}
+```
+
+## Install via Nix/Home Manager
 
 `lazy-lsp-nvim` is available in the unstable channel. Install with: