Recommend NPM instead of Yarn (#1229)

Author: nicolas-grekas

api-platform/admin-pack/1.0/post-install.txt

@@ -1,5 +1,5 @@
   * Your admin is almost ready:
-    1. Install the JavaScript dependencies by running <comment>yarn add @api-platform/admin @babel/preset-react</comment>
+    1. Install the JavaScript dependencies by running <comment>npm add @api-platform/admin @babel/preset-react</comment>
     2. Edit <comment>webpack.config.js</comment> and <info>uncomment</info> the following lines:
 
         Encore

symfony/webpack-encore-bundle/2.0/post-install.txt

@@ -1,3 +1,5 @@
-  * Install Yarn and run <fg=green>yarn install</>
+  * Install NPM and run <fg=green>npm install</>
 
-  * Start the development server: <fg=green>yarn encore dev-server</>
+  * Compile your assets: <fg=green>npm run dev</>
+
+  * Or start the development server: <fg=green>npm run watch</>