Add explanation of how to view a rosbag

[lucasw]

Summary

I couldn't figure out how to load a rosbag until coming across #161

Test plan

I ran npm run docs and browsed to localhost:8080/try, then dragged in a rosbag.

Versioning impact

No.

[jtbandes]

FYI, we also have this in the UI already:

[lucasw]

I knew about the drag n drop part, it was running 'docs' and changing the url to get to the page that allows it that isn't obvious. Having something like npm run webviz-core or npm run view-rosbag or similar would be nice to skip the other steps.

[jtbandes]

A lot of this info is also in the main readme: https://github.com/cruise-automation/webviz/blob/master/README.md#developing So I wonder if we can link there instead so they don't get out of date?