docs: add the contribution steps to semi-icon package (#1246)

* docs: add the contribution steps to semi-icon package

* docs: add the contribution steps to semi-icon package

packages/semi-icons/CONTRIBUTING.md

@@ -0,0 +1,9 @@
+## 📣 Introduction
+
+How to contribute icon to this package
+
+##  🚀 Steps
+- Add the svg file under the path ```src/svgs/```, naming references to other files.
+- Run command ```lerna run build:icon --scope @douyinfe/semi-icons```, the corresponding tsx file will be generated under the path ```src/icons```.
+- Add the category and name of the new icon to the ```src/svgs/meta.json``` file.
+- Finally, run command ```yarn docsite``` and go to the icon page to confirm whether the icon is successfully added.