docs: update README

demo/README.md

@@ -0,0 +1,43 @@
+## Using `nvim-lspconfig`
+
+The specification is not stable, so you need to set it yourself. Once the spec is stable, I will send a PR to `nvim-lspconfig`.
+```
+local lspconfig = require('lspconfig')
+local configs = require('lspconfig.configs')
+local util = require "lspconfig/util"
+
+configs.testing_ls = {
+  default_config = {
+    cmd = { "testing-language-server" },
+    filetypes = { "rust" },
+    root_dir = util.root_pattern(".git", "Cargo.toml"),
+      init_options = {
+        enable = true,
+        fileTypes = {"rust"},
+        adapterCommand = {
+          rust = {
+            {
+              path = "testing-ls-adapter",
+              extra_arg = { "--test-kind=cargo-test", "--workspace" },
+              include = { "/demo/**/src/**/*.rs"},
+              exclude = { "/**/target/**"},
+            }
+          }
+        },
+        enableWorkspaceDiagnostics = true,
+        trace = {
+          server = "verbose"
+        }
+      }
+  },
+  docs = {
+    description = [[
+      https://github.com/kbwo/testing-language-server
+
+      Language Server for real-time testing.
+    ]],
+  },
+}
+
+lspconfig.testing_ls.setup{}
+```