Add missing build doc

CONTRIBUTING.md

@@ -0,0 +1,21 @@
+# Compiling and Building
+
+We use NodeJS 14 to build skywalking-nodejs project, if you don't have NodeJS 14 installed,
+you can choose a node version manager like [nvm](https://github.com/nvm-sh/nvm) to easily
+manage multiple node vesions, or you can start a Docker container and build this project inside
+the container.
+
+```shell
+# Suppose you have the source codes in folder skywalking-nodejs
+docker run -it --rm -v $(pwd)/skywalking-nodejs:/workspace -w /workspace node:14 bash
+```
+
+Then run the following commands to build the project:
+
+```shell
+npm install
+npm run build
+```
+
+Warnings can be ignored, but if you have any error that prevents you to continue, try
+`rm -rf node_modules/` and then rerun the commands above.