Add contributing file

Fixes #836

Author: Tyriar

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+## Testing in a real terminal
+
+The recommended way to test node-pty during development is to clone the xterm.js repository and link's node-pty module to this directory.
+
+1. Clone xterm.js in a separate folder:
+    ```sh
+    git clone https://github.com/xtermjs/xterm.js
+    npm ci
+    npm run setup
+    ```
+2. Link the node-pty repo:
+    ```
+    rm -rf node_modules/node-pty # in xterm.js folder
+    ln -s <path_to_node-pty> <path to xtermjs>/node_modules/node-pty
+    ```
+3. Hit ctrl/cmd+shift+b in VS Code or run the build/demo scripts manually:
+    ```sh
+    npm run tsc-watch # build ts
+    npm run esbuild-watch # bundle ts/js
+    npm run esbuild-demo # build demo/server
+    npm run start # run server
+    ```
+4. Open http://127.0.0.1:3000 and test
+5. Kill and restart the `npm run start` command to apply any changes made in node-pty