docs: update CONTRIBUTING.md (vuejs#2493)

Author: ulivz

.github/CONTRIBUTING.md

@@ -4,17 +4,14 @@ This project uses a monorepo setup that requires using [Yarn](https://yarnpkg.co
 
 ``` sh
 # Install all dependencies.
-yarn install
+yarn
 
 # Serves VuePress' own docs with itself.
 yarn dev
 
 # Build VuePress' own docs with itself.
 yarn build
 
-# Execute all the test suites.
-yarn test
-
 # Clean dependencies.
 yarn clean
 
@@ -24,7 +21,15 @@ yarn boot
 
 ## Testing Setup
 
-> TODO
+VuePress leverages [jest](https://jestjs.io/) for its tests, testing process depends on some setup located at [scripts/test.js](../scripts/test.js).
+
+```bash
+# Execute all the test suites.
+yarn test
+
+# Execute tests under specfic package. 
+yarn test -p=core ## OR --package=core
+```
 
 ## Core Packages
 

scripts/test.js

@@ -8,7 +8,7 @@ const args = minimist(rawArgs)
 let regex
 const debug = !!args['inspect-brk']
 
-if (args.p) {
+if (args.p || args.package) {
   const packages = (args.p || args.package).split(',').join('|')
   regex = `.*@vuepress/(${packages}|plugin-(${packages}))/.*\\.spec\\.(js|ts)$`
   const i = rawArgs.indexOf('-p')