Adding documentation for SDMA environment var

Adding documentation for modifiers for SDMA copy

Change-Id: I2425672c3ba1f1617d29b8f4b49776775d78a376

runtime/hsa-runtime/README.md

@@ -214,6 +214,26 @@ which implement HSA functionality for ROCm supported devices.
 * `hsa_system_get_extension_table` is not implemented for
   `HSA_EXTENSION_AMD_PROFILER`.
 
+## Environment Variables
+
+### HSA_ENABLE_SDMA
+Possible values:
+* 0:Disabled
+* 1:Enabled (Default Value)
+
+This will enable or disable the use of DMA engines in all copy directions (Host-to-Device, Device-to-Host, Device-to-Device) when using the following APIs:
+`hsa_memory_copy`, `hsa_amd_memory_fill`, `hsa_amd_memory_async_copy`, `hsa_amd_memory_async_copy_on_engine`
+
+### HSA_ENABLE_PEER_SDMA
+Possible values:
+* 0:Disabled
+* 1:Enabled (Default Value)
+
+This will enable or disable the use of DMA engines for Device-to-Device copies when using the following APIs:
+`hsa_memory_copy`, `hsa_amd_memory_async_copy`, `hsa_amd_memory_async_copy_on_engine`
+
+The value of `HSA_ENABLE_PEER_SDMA` is ignored if `HSA_ENABLE_SDMA` is used to disable the use of DMA engines.
+
 ## Disclaimer
 
 The information contained herein is for informational purposes only, and is