Update READMEs for yarn (#8215) [skip ci]

Closes https://github.com/babel/babel/pull/8255

scripts/generators/readmes.js

@@ -54,13 +54,13 @@ See our website [${name}](${websiteLink}) for more information${
 Using npm:
 
 \`\`\`sh
-npm install --save ${name}
+npm install --save-dev ${name}
 \`\`\`
 
 or using yarn:
 
 \`\`\`sh
-yarn add --save ${name}
+yarn add ${name} --dev
 \`\`\`
 `;