Added guidelines for contributing

.github/CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Before asking for assistance
+
+Before asking for help, please read the [FAQ](https://bblanchon.github.io/ArduinoJson/faq/).
+
+If you did not find the answer in the FAQ, please create a new issue.
+
+It is OK to add a comment to a currently opened issue, but please avoid adding comments to a closed issue.
+
+# Before hitting the Submit button
+
+Please provide all the relevant information:
+
+* Good title
+* Short description of the problem
+* Target platform
+* Compiler model and version
+* [MVCE](https://stackoverflow.com/help/mcve)
+* Compiler output
+
+Good questions get fast answers!
+
+# Pull requests
+
+Please:
+
+1. Unit test every change in behavior
+2. Use clang-format in "file" mode to format the code
+3. Consider using the Continuous Integration (Travis and AppVeyor)
+