[DOCS] (NMIND gold) Continuous integration badges in README for coverage

[zhao-cy]

i.e., to set up Codecov for BABS.

[zhao-cy]

Instructions on how to set it up: https://app.codecov.io/gh/PennLINC/babs/new

We need to set it up in CircleCI where tests are run, instead of in github workflow instructed by codecov. From @tsalo:

Each step that runs tests in the CircleCI config file is set up to make a coverage file. Those coverage files are then stored across steps, so they can be loaded, merged, and uploaded to CodeCov in a separate step in the CircleCI config. See https://github.com/PennLINC/xcp_d/blob/59fe82d839a62b9f91161c93721dbf94dc9c659e/.circleci/config.yml#L362-L378 for the step that merges and uploads the coverage file.