Update relationships between Assertions and Evidence Lines in figure in readme

[bpow]

The figure in the readme (https://github.com/monarch-initiative/SEPIO-ontology/blob/master/docs/SEPIO%20Wiki%201.jpg) is not in line with current thoughts on the overall model (at least with regard to discussions regarding the ClinGen interpretation model.

Two things that immediately come to mind:

1. The diagram only shows Assertions being related to Evidence Lines through Assertion Process nodes (which you've discussed eliminating from the model.
2. The relationships in point 1 above are each one-to-one, which lead to an implicit 1 to 1 relationship between an Assertion and an Evidence Line, but my understanding is that an assertion may be supported by multiple Evidence Lines