warn before running function builder

src/commands/dev/index.js

@@ -232,6 +232,16 @@ class DevCommand extends Command {
     if (functionsDir) {
       const functionBuilder = await detectFunctionsBuilder(settings);
       if (functionBuilder) {
+        this.log(
+          `${NETLIFYDEVLOG} Function builder ${chalk.yellow(
+            functionBuilder.builderName
+          )} detected: Running npm script ${chalk.yellow(
+            functionBuilder.npmScript
+          )}`
+        );
+        this.warn(
+          `${NETLIFYDEVWARN} This is a beta feature, please give us feedback on how to improve at https://github.com/netlify/netlify-dev-plugin/`
+        );
         await functionBuilder.build();
         const functionWatcher = chokidar.watch(functionBuilder.src);
         functionWatcher.on("add", functionBuilder.build);

src/function-builder-detectors/netlify-lambda.js

@@ -35,6 +35,7 @@ module.exports = function() {
   if (settings.npmScript) {
     settings.build = () =>
       execa(yarnExists ? "yarn" : "npm", ["run", settings.npmScript]);
+    settings.builderName = "netlify-lambda";
     return settings;
   }
 };