Merge pull request #47 from OpenBioSim/feature_CUDA_docs

Adds CUDA documentation to README

README.md

@@ -57,6 +57,14 @@ An example perturbable system for a methane to ethanol perturbation in solvent
 can be found [here](https://sire.openbiosim.org/m/merged_molecule.s3.bz2).
 This is a `bzip2` compressed file that will need to be extracted before use.
 
+#### Running SOMD2 using one or more GPUs
+
+In order to run using GPUs you will first need to set the relevant environment variable. For example, to run using 4 CUDA enabled GPUS set `CUDA_VISIBLE_DEVICES=0,1,2,3` (for openCL and HIP use `OPENCL_VISIBLE_DEVICES` and `HIP_VISIBLE_DEVICES` respectively). 
+
+By default SOMD2 will run using the CPU platform, however if the relevant environment variable has been set (as above) the new platform will be detected and set. In the case that this detection fails, or if there are multiple platforms available, the `--platform` option can be set (for example `--platform cuda`).
+
+By default, SOMD2 will automatically manage the distribution of lambda windows across all listed devices. In order to restrict the number of devices used the `--max_gpus` option can be set, for example setting `max_gpus=2` while `CUDA_VISIBLE_DEVICES` are set as above would restrict SOMD2 to using only GPUs 0 and 1.
+
 ## Analysis
 
 Simulation output will be written to the directory specified using the