Add contribution guidelines.

CONTRIBUTING.md

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+### Pull request policy
+Fix one issue per pull request.  
+Squash commits before opening a pull request.  
+Respect the coding style policy.    
+Name your PR like the following :
+
+* When correcting a single bridge, use `[BridgeName] Feature`.  
+* When fixing a problem in a specific file, use `[FileName] Feature`.
+* When fixing a general problem, use `category : feature`.
+
+Note that all pull-requests should pass the unit tests before they can be merged.  
+
+### Coding style
+
+Use `camelCase` for variables and methods.  
+Use `UPPERCASE` for constants.  
+Use `PascalCase` for class names. When creating a bridge, your class and PHP file should be named `MyImplementationBridge`.   
+Use tabs for indentation.  
+Add an empty line at the end of your file.  
+
+Use `''` to encapsulate strings, including in arrays.  
+Prefer lines shorter than 80 chars, no line longer than 120 chars.  
+PHP constants should be in lower case (`true, false, null`...)  
+
+
+* Add spaces between the logical operator and your expressions (not needed for the `!` operator).  
+* Use `||` and `&&` instead of `or` and `and`.  
+* Add space between your condition and the opening bracket/closing bracket.  
+* Don't put a space between `if` and your bracket.  
+* Use `elseif` instead of `else if`.  
+* Add new lines in your conditions if they are containing more than one line.  
+*  Example :  
+
+```PHP
+if($a == true && $b) {
+  print($a);
+} else if(!$b) {
+
+  $a = !$a;
+  $b = $b >> $a;
+  print($b);
+
+} else {
+  print($b);
+}
+```
+