Suggest add some more pull request guidance in contributing

[kwanhur]

Like some appoints:

1. One issue just open one pull request, even modify with other ways.

   This avoids too much pull request notices will be pushed to reviewer's inbox like a flood.

2. After picking one issue try to resolve, advice to comment the complete solution.

   So that, the owner or other will discuss it and more opinions will come out.

3. etc...

Show out the friendly way to participate in. Any other ideas are welcome.