chore: add README with instructions for installing (#1)

Author: sam

README.md

@@ -1 +1,29 @@
-# replace this
+# Functionless Language Service Plugin
+
+[![npm version](https://badge.fury.io/js/@functionless%2Flanguage-service.svg)](https://badge.fury.io/js/@functionless%2Flanguage-service)
+
+The `@functionless/language-service` package configures a plugin for the TypeScript language service plugin that adds real-time errors to the IDE for [Functionless](https://github.com/functionless/functionless)'s semantic errors.
+
+To configure, first install the module as a devDependency.
+
+```shell
+# if using NPM
+npm install --save-dev @functionless/language-service
+
+# if using yarn
+yarn add -D @functionless/language-service
+```
+
+Then add as a plugin to your `tsconfig.json`.
+
+```json
+{
+  "compilerOptions": {
+    "plugins": [
+      {
+        "name": "@functionless/language-service"
+      }
+    ]
+  }
+}
+```