Add OpenGL section.

Add modern, working and actively maintained libraries that are needed to create OpenGL applications in Go.
Added Uniqush-push
typos
Add goxc
Merge pull request #55 from avelino/add-opengl-section

Add OpenGL section.
Merge pull request #56 from vincent/master

Add a messaging category
Merge pull request #57 from mattn/goxc

Add goxc
Add gotk3 to GUI section
Merge pull request #60 from dom3k/master

Add gotk3 to GUI section
added go-webcolors library
Merge pull request #61 from jyotiska/master

Added go-webcolors library
Add Testify on Testing section.
Add amber on Template Engine section.
remove duplicate snowball #62
Merge pull request #63 from romanoaugusto88/add_testify

Add Testify on Testing section.
Merge pull request #64 from romanoaugusto88/add_amber

Add amber on Template Engine section.
Added gomail and stdlog
Merge pull request #65 from alexcesaro/master

Added gomail and stdlog
New draft of the contribution guidelines
Moved contribution link to the top