add pointer from README to extensions folder

Signed-off-by: Jeff Hammond <jeff.r.hammond@intel.com>

Author: Jeff Hammond

README.md

@@ -17,3 +17,7 @@ See [CONTRIBUTING.md](https://github.com/intel/llvm/blob/intel/CONTRIBUTING.md)
  - SYCL Compiler and Runtimes - See [GetStartedWithSYCLCompiler.md](https://github.com/intel/llvm/blob/sycl/sycl/doc/GetStartedWithSYCLCompiler.md)
 
 *Other names and brands may be claimed as the property of others.
+
+## SYCL Extension Proposal Documents
+
+See [sycl/doc/extensions](https://github.com/intel/llvm/tree/sycl/sycl/doc/extensions)