chore: add CONTRIBUTING.md (#4)

Author: cwaldren-ld

CONTRIBUTING.md

@@ -0,0 +1,80 @@
+## Contribution Guide
+
+### Formatting
+
+Slightly modified "Chromium" `clang-tidy` configuration.
+
+### Style
+
+See https://google.github.io/styleguide/cppguide.html. 
+
+Quick start:
+
+**Class member variables**
+```c++
+class Thing {
+    std::string foo_;
+    std::string bar_baz_;
+};
+```
+**Struct member variables**
+```c++
+struct Thing {
+    std::string foo;
+    std::string bar_baz;
+};
+```
+**Class names**
+```c++
+class Foo;
+class BarBaz;
+```
+**Functions**
+```c++
+DoTheFoo();
+Bar();
+```
+**Constants**
+```c++
+const int kFooBarBaz = 1;
+```
+**Enums**
+```C++
+enum class Foo {
+    kUnknown = 0,
+    kBar,
+};
+```
+
+**Divergence: file names**
+
+In C:
+```
+file_name.c
+file_name.h
+```
+In C++:
+```
+file_name.cpp
+file_name.hpp
+```
+### C++ Standard
+
+C++17.
+
+### C Standard
+
+C99. 
+
+### Testing
+
+Use `googletest`.
+
+### Exporting
+
+Symbols should be hidden by default. Only export what is part of the public API.
+
+
+### Include Guards
+
+Use `#pragma once`.