add install hint for OpenMP on Mac

This was necessary for me to run the images page in the dashboard. On the command line, I got this message:

XGBoostError:
XGBoost Library (libxgboost.dylib) could not be loaded.
Likely causes:
  * OpenMP runtime is not installed
    - vcomp140.dll or libgomp-1.dll for Windows
    - libomp.dylib for Mac OSX
    - libgomp.so for Linux and other UNIX-like OSes
    Mac OSX users: Run `brew install libomp` to install OpenMP runtime.

  * You are running 32-bit Python on a 64-bit OS

Author: egpbos

README.md

@@ -101,6 +101,7 @@ python3 -m pip install git+https://github.com/dianna-ai/dianna.git
 ```
 
 If you get an error related to OpenMP when importing dianna, have a look at [this issue](https://github.com/dianna-ai/dianna/issues/376) for possible workarounds.
+For some usecases (e.g. on Intel Macs), you may need to install OpenMP separately, e.g. with `brew install libomp`.
 
 <details><summary>Pre-requisites only for Macbook Pro with M1 Pro chip users</summary>
 <p>