Merge branch 'opencl-in-readme' into 'master'

Mention `opencl` option in README.md

See merge request pycodegen/lbmpy!28

README.md

@@ -37,7 +37,8 @@ pip install lbmpy[interactive]
 Without `[interactive]` you get a minimal version with very little dependencies.
 
 All options:
--  `gpu`: use this if nVidia GPU is available and CUDA is installed
+- `gpu`: use this if nVidia GPU is available and CUDA is installed
+- `opencl`: use this to enable the target `opencl` (execution using OpenCL)
 - `alltrafos`: pulls in additional dependencies for loop simplification e.g. libisl
 - `interactive`: installs dependencies to work in Jupyter including image I/O, plotting etc.