Move the pull request guide to the developer guide
Move the pull request guide to the developer guide

Motivation:

CONTRIBUTING.md is useful only because it lets Github show a user the
link to it so the user can check what information we need before
submitting a bug report.  However, Github does not do the same for a
pull request submission form, and thus there's no reason to keep the
information about how to submit a good pull request in CONTRIBUTING.md.

Modification:

Replace the section about issuing a pull request with the link to the
official developer guide.

Result:

CONTRIBUTING.md is easier to maintain.

DavidAlphaFox/netty,DavidAlphaFox/netty,imangry/netty-zh,yipen9/netty,imangry/netty-zh,chanakaudaya/netty,imangry/netty-zh,yipen9/netty,DavidAlphaFox/netty,purplefox/netty-4.0.2.8-hacked,purplefox/netty-4.0.2.8-hacked,chanakaudaya/netty,yipen9/netty,chanakaudaya/netty,chanakaudaya/netty,purplefox/netty-4.0.2.8-hacked,imangry/netty-zh,chanakaudaya/netty,DavidAlphaFox/netty,yipen9/netty,purplefox/netty-4.0.2.8-hacked,imangry/netty-zh