Added CONTRIBUTING.md, ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md

for Github.

CONTRIBUTING.md

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+Contribution Guidelines
+=======================
+
+Coding Standards
+----------------
+
+The code is written to follow 
+[PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md) standards.

ISSUE_TEMPLATE.md

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+Expected behaviour
+------------------
+
+*Please describe what you're expecting to see happen.*
+
+Actual behaviour
+----------------
+
+*Please describe what you're actually seeing happen.*
+
+Steps to reproduce
+------------------
+
+*If your issue requires any specific steps to reproduce, please outline them here.*
+
+Environment info
+----------------
+
+*OS, PHP version, MySQL/PostgreSQL version.*

PULL_REQUEST_TEMPLATE.md

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+Contribution Typ
+----------------
+
+*Bug fix, new plugin, feature, ui etc.*
+
+Description of change
+---------------------
+
+*Detailed description of change.*