{Releasing} Add step for `.gitattributes` to documentation. (#6352)
{Releasing} Add step for `.gitattributes` to documentation. (#6352)

Adding a new "setup" step for release engineers to run before cutting the
release. This step will be important to avoid merging changes to
`.gitattributes` from `develop` into `stable` and overwriting the custom
values there.

Documentation for #6350
Follow-up to #5956
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios