Add guideline for HIP_INIT_API

Change-Id: Ib8fa1e952f31158e9435dfc37ff23db9fda6fd7e

CONTRIBUTING.md

@@ -117,6 +117,10 @@ Differences or limitations of HIP APIs as compared to CUDA APIs should be clearl
     - Keyword TODO refers to a note that should be addressed in long-term.  Could be style issue, software architecture, or known bugs.
     - FIXME refers to a short-term bug that needs to be addressed.
 
+- HIP_INIT_API() should be placed at the start of each top-level HIP API.  This function will make sure the HIP runtime is initialized,
+  and also constructs an appropriate API string for tracing and CodeXL marker tracing.  The arguments to HIP_INIT_API should match
+  those of the parent fucntion.  
+
 
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 Before checking in or submitting a pull request, run all Rodinia tests and ensure pass results match starting point: