Document usage of `continue-on-error` for pipeline usage

[shrink]

A pipeline might derive value from the result of passed (e.g: checking if a dist directory exists in order to allow a tag to be created). The continue-on-error option enables this use-case.

- id: is-built
  continue-on-error: true
  uses: pr-mpt/actions-assert@v2
  with:
    assertion: "npm://@assertions/directory-exists"
    actual: "dist"

[shrink]

#56 should be the focus rather than continue-on-error which should be avoided because it ignores multiple types of error.