Add a Gitter chat badge to README.md

[gitter-badger]

luca-piccioni/OpenGL.Net now has a Chat Room on Gitter

@luca-piccioni has just created a chat room. You can visit it here: https://gitter.im/OpenGL-Net/Lobby.

This pull-request adds this badge to your README.md:

If my aim is a little off, please let me know.

Happy chatting.

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[codecov]

Codecov Report

Merging #94 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files          95       95           
  Lines       27496    27496           
  Branches     1946     1946           
=======================================
  Hits        25254    25254           
  Misses       2000     2000           
  Partials      242      242

Flag	Coverage Δ
#WGL_net35_x64_Release	92.14% <ø> (ø)	⬆️
#WGL_net35_x86_Release	90.61% <ø> (ø)	⬆️
#WGL_net461_x64_Release	91.53% <ø> (ø)	⬆️
#WGL_net461_x86_Release	90.64% <ø> (ø)	⬆️

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[ghost]

I join our gitter now and I want tell about OpenGL.net is really sexy and perfect because OpenTK's viewport can not load since application's start. OpenTK is loser, OpenGL.net is winner. I need learn more because OpenTK and OpenGL.net are very different. Thanks for sharing and you can see my youtube on gitter. :)