Document migration from old bats-assert

[jasonkarns]

Add note when complete: ztombol#20 ztombol#21

[jasonkarns]

• assert, refute, assert_equal, assert_output should all work as before

• flunk -> fail (from bats-support)

• assert_success with optional output argument -> assert_success; assert_output

• assert_failure with optional output argument -> assert_failure; assert_output

• assert_contains -> 🤷‍♂️ (for output, can use assert_output --partial)

• refute_contains -> 🤷‍♂️ (for output, can use refute_output --partial)

• assert_starts_with -> (for output, can use assert_output --regex '^x')

• assert_output_contains -> assert_output --partial

• refute_output_contains -> refute_output --partial

• assert_line with optional index -> assert_line --index N

• refute_line with optional index -> refute_line --index N

[jasonkarns]

https://github.com/jasonkarns/bats-assert-1/wiki/Migrating-from-v1.1.1