add Readme.md to Quda benchmark

Quda/Readme.md

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+# QUDA benchmarks
+
+This folder contains benchmarks for the [QUDA](https://github.com/lattice/quda) library.
+
+- `Benchmark_Quda`: This benchmark measure floating point performances of fermion
+matrices (Wilson and DWF), as well as memory bandwidth (using a simple `axpy` operation). Measurements are
+performed for a fixed range of problem sizes.
+
+## Building
+After setting up your compilation environment (Tursa: `source /home/dp207/dp207/shared/env/production/env-{base,gpu}.sh`):
+```bash
+./build-quda.sh <env_dir>          # build Quda
+./build-benchmark.sh <env_dir>     # build benchmark
+```
+where `<env_dir>` is an arbitrary directory where every product will be stored.
+
+## Running the Benchmark
+
+The benchmark should be run as
+```bash
+mpirun -np <ranks> <env_dir>/prefix/qudabench/Benchmark_Quda
+```
+where `<ranks>` is the total number of GPU's to use. On Tursa this is 4 times the number of nodes.
+
+Note: on Tursa, the `wrapper.sh` script that is typically used with Grid is not necessary.