Add contributing guideline

.github/CONTRIBUTING.md

@@ -0,0 +1,54 @@
+Contribution Guidelines
+=======================
+
+Contributions to cJSON are welcome. If you find a bug or want to improve cJSON in another way, pull requests are appreciated.
+
+For bigger changes, in order to avoid wasted effort, please open an issue to discuss the technical details before creating a pull request.
+
+The further sections explain the process in more detail and provides some guidelines on how contributions should look like.
+
+Branches
+--------
+There are two branches to be aware of, the `master` and the `develop` branch. The `master` branch is reserved for the latest release, so only make pull requests to the `master` branch for small bug- or security fixes (these are usually just a few lines). In all other cases, please make a pull request to the `develop` branch.
+
+Coding Style
+------------
+The coding style has been discussed in [#24](https://github.com/DaveGamble/cJSON/issues/24). The basics are:
+
+* Use 4 tabs for indentation
+* No oneliners (conditions, loops, variable declarations ...)
+* Always use parenthesis for control structures
+* Don't implicitly rely on operator precedence, use round brackets in expressions. e.g. `(a > b) && (c < d)` instead of `a>b && c<d`
+* opening curly braces start in the next line
+* use spaces around operators
+* lines should not have trailing whitespace
+* use spaces between function parameters
+* use pronouncable variable names, not just a combination of letters
+
+Example:
+
+```c
+/* calculate the new length of the string in a printbuffer and update the offset */
+static void update_offset(printbuffer * const buffer)
+{
+    const unsigned char *buffer_pointer = NULL;
+    if ((buffer == NULL) || (buffer->buffer == NULL))
+    {
+        return;
+    }
+    buffer_pointer = buffer->buffer + buffer->offset;
+
+    buffer->offset += strlen((const char*)buffer_pointer);
+}
+```
+
+Unit Tests
+----------
+cJSON uses the [Unity](https://github.com/ThrowTheSwitch/Unity) library for unit tests. The tests are located in the `tests` directory. In order to add a new test, either add it to one of the existing files (if it fits) or add a new C file for the test. That new file has to be added to the list of tests in `tests/CMakeLists.txt`.
+
+All new features have to be covered by unit tests.
+
+Other Notes
+-----------
+* Internal functions are to be declared static.
+* Wrap the return type of external function in the `CJSON_PUBLIC` macro.