Insert 'yarn' command between 'cd storybook' and 'yarn bootstrap (--c…

…ore)'

Signed-off-by: Yeseul Jeon <yeseouler52@gmail.com>

CONTRIBUTING.md

@@ -27,6 +27,7 @@ To test your project against the current latest version of storybook, you can cl
 ```sh
 git clone https://github.com/storybookjs/storybook.git
 cd storybook
+yarn
 yarn bootstrap
 ```
 
@@ -138,6 +139,7 @@ A good way to do that is using the example `cra-kitchen-sink` app embedded in th
 # Download and build this repository:
 git clone https://github.com/storybookjs/storybook.git
 cd storybook
+yarn
 yarn bootstrap --core
 
 # make changes to try and reproduce the problem, such as adding components + stories