Create document to summarize the code that I have added for SSL

[jeffreydominic]

• One objective of this document is to describe how a user would use a particular function. For example, I will describe the different ways a user can load pretrained weights using the code in load_weights_utils.py.
• The other objective is to summarize the changes that I made in MedSegPy, so we can tell what needs to be transferred to the original MedSegPy repo
• ETA: 04/22/22 (Updated ETA: 04/25/22)