Create CONTRIBUTING.md (#54)

CONTRIBUTING.md

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+# Quick style guide
+
+* Braces on a new line
+* Macros and return types on a separate line from the function definition:
+  ```cpp
+  HINLINE float
+  HMM_MyFunction()
+  {
+      // ...
+  }
+  ```
+* Explicitly initialize variables to zero:
+  ```cpp
+  HINLINE float
+  HMM_MyFunction()
+  {
+      float MyFloat = 0.0f;
+      hmm_vec3 MyVector = {0};
+  }
+  ```
+* Parentheses around the returned value:
+  ```cpp
+  HINLINE float
+  HMM_MyFunction()
+  {
+      return (1.0f);
+  }
+  ```
+
+
+# Other notes
+
+* If a new function is defined with different names for different datatypes, also add C++ overloaded versions of the functions. For example, if you have `HMM_LengthVec2(hmm_vec2)` and `HMM_LengthVec3(hmm_vec3)`, also provide `HMM_Length(hmm_vec2)` and `HMM_Length(hmm_vec3)`.
+
+  It is fine for the overloaded versions to call the C versions.
+* Only use operator overloading for operations already built into C. That means no using `^` for cross product or `|` for dot product.
+* Try to define functions in the same order as the prototypes.
+* Don't forget that Handmade Math uses column-major order for matrices!
+