title: How to send pull request from my fork to another fork?
body: <p>I forked a repository <code>R</code> as <code>R1</code>. Then I make some changes to <code>R1</code>.</p>
  
  <p>B forked the repository <code>R</code> as <code>R2</code>, and <code>R2</code> becomes the mainly maintained repository.</p>
  
  <p>Now I want to send pull request to <code>R2</code>, how to do?</p>
  
  <p>And what if I want to keep my <code>R1</code> updated with <code>R2</code>?</p>