update readme installation instructions to use pip, add a section abo…

[WilsonGregory]

Changes

• none

Testing

• inspected the markdown file

Doc Changes

• update the installation to use pip
• old installation instructions are moved to the For Developers section since they will still need them for an editable install
• add instructions about how to add your own models and benchmark them

[beelze-b]

I really like the changes. The notes are excellent. You've done an incredible job at making the package accessible!