add function builder readme

Author: swyxio

src/function-builder-detectors/README.md

@@ -0,0 +1,16 @@
+## function builder detectors
+
+similar to project detectors, each file here detects function builders. this is so that netlify dev never manages the webpack or other config. the expected output is very simple:
+
+```js
+module.exports = {
+  src: "functions-source", // source for your functions
+  build: () => {}, // chokidar will call this to build and rebuild your function
+  npmScript: "build:functions" // optional, the matching package.json script that calls your function builder
+}
+```
+
+example
+
+- [src](https://github.com/netlify/netlify-dev-plugin/blob/6a3992746ae490881105fbed2e11ca444f79e44e/src/function-builder-detectors/netlify-lambda.js#L29)
+- [npmScript](https://github.com/netlify/netlify-dev-plugin/blob/6a3992746ae490881105fbed2e11ca444f79e44e/src/function-builder-detectors/netlify-lambda.js#L30)