Include `isostasy` in the doc of `yelmo`

So far,  the doc of yelmo(x) does not include sufficient information to use `isostasy` properly. We should add something like the lines below:

```
git clone https://github.com/palma-ice/isostasy
cd isostasy
python3 config.py config/pik_ifort
ln -s path_to/IsostasyData 
```
IsostasyData can be downloaded by running:

```
git clone https://github.com/JanJereczek/IsostasyData
```