added prs welcome (from shields.io)

- signed-off-by: trimstray <trimstray@gmail.com>

README.md

@@ -12,6 +12,10 @@
     <img src="https://img.shields.io/badge/Branch-master-green.svg?longCache=true"
         alt="Branch">
   </a>
+  <a href="https://github.com/trimstray/the-book-of-secret-knowledge/pulls">
+    <img src="https://img.shields.io/badge/PRs-welcome-brightgreen.svg?longCache=true"
+        alt="Pull Requests">
+  </a>
   <a href="http://www.gnu.org/licenses/">
     <img src="https://img.shields.io/badge/License-GNU-blue.svg?longCache=true"
         alt="License">