EDIT: For newer versions of QGIS (e.g. 3.12+) see "EDIT" below.
I have utilised the above link that @Fran supplied to add in the appropraite paths to my interpreter without cloning the repository. I have added the following paths to my interpreter:
Needed for processing framework imports
It was a bit weird trying to find where to add the paths in PyCharm
Preferences >> Project Interpreter >> Click the down arrow to choose interpreter >> Show All> Select the appropriate interpreter>> then click on the folder (Show paths for the selected interpreter >> Add the paths >> apply.
For newer versions of QGIS (e.g. 3.12+) the path to the interpreter (change to 'QGIS-LTR.app' if you are using the long term release) is as so:
When adding the new new interpreter, check 'Inherit...' to make sure the QGIS relevant packages are included.