Add RFC for rocFFT OpenCL support

text/0000-rocFFT-OpenCL.md

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+- Feature Name: rocFFT OpenCL support 
+- Start Date: 2018-05-15 
+- RFC PR: (leave this empty)
+- ROCm Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+To support the OpenCL community, the rocFFT library should provide
+means for it to be used in OpenCL projects.
+
+# Motivation
+[motivation]: #motivation
+
+FFTs are often used components in scientific and engineering computing
+and therefore there is a broad set of use-cases where computing FFTs on
+ROCm platforms would be of great benefit (e.g. GROMACS, OpenMM / Folding@HOME, etc.)
+
+# Detailed design
+[design]: #detailed-design
+
+* Short-term solution: use `clCreateProgramWithBinary` to load precompiled FFT kernels;
+* Longer-term more complete solution: implemet the `rocfft.h` API for OpenCL;
+* Device-side callable 1D FFT kernels would also be very valuable to be
+developed for release when the required OpenCL support also lands in a future ROCm
+stack release.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Effort may be better invested in clFFT and its compatibility with the ROCm toolchain
+as well as early high quality and high performance support for recent AMD hardware.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Other designs: -
+Impact of reject: No high performance FFT will be available for portable GPU applications on AMD GPUs.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+The detailed design is really up to the rocFFT and OpenCL runtime's dev.
+