Add README

README.md

@@ -0,0 +1,28 @@
+# Pixel raw2digi test program
+
+The purpose of this test program is to experiment with various
+"performance portability" approaches.
+
+## Current implementations
+
+| Implementation | Execuable    | `make` target | `#ifdef` macro |
+|----------------|--------------|---------------|----------------|
+| Naive CPU      | `main-naive` |`naive`        | `DIGI_NAIVE`   |
+| CUDA           | `main-cuda`  |`cuda`         | `DIGI_CUDA`    |
+
+### Naive CPU
+
+The only requirements for "naive CPU" are `g++` supporting C++ 17 in the `$PATH`.
+
+### CUDA
+
+The CUDA test program requires a recent CUDA version (`nvcc`
+supporting C++14 and `--expt-relaxed-constexpr`) and a machine with
+GPU.
+
+## How to add a new implementation?
+
+- Copy of (e.g.) the `rawtodigi_naive.h` for the new *thing* (with name of the *thing* after the underscore)
+- Enclose all specifics under `#ifdef DIGI_THING`
+- Add new build rules to `Makefile`
+- Update `README.md`, including the requirements to compile and run with the *thing*