Adding contributing guide

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Version 1.0 (in progress)
+* Cleanup using `clang-tidy` and `clang-format`
+* Small improvements to Timers, easier to subclass Error
+
 ## Version 0.9
 
 * Better CMake named target (CLI11)

CONTRIBUTING.md

@@ -0,0 +1,9 @@
+#A few notes on contributions
+
+If you want to add code, please make sure it passes the clang-format style (I'm using LLVM 4.0):
+
+```bash
+git ls-files | grep -E '.cpp|.hpp' | xargs clang-format -i -style=file
+```
+
+It is also a good idea to check this with `clang-tidy`; automatic fixes can be made using `-DCLANG_TIDY_FIX-ON` (resets to `OFF` when rerunning CMake).