Update README to clarify rules about stdout/sterr
Update README to clarify rules about stdout/sterr

A short note in the README to explain that users should not write anything
to stdout outside of @test, setup or teardown functions.

Also added a link pointing to the wiki where more information is given
about how Bats evaluates test files.

Sylvain303/bats,mjhsieh/bats,rvesse/bats,thomasleveil/bats,msabramo/bats,bpkg/bats,jszwedko/bats,Anvil/bats,sstephenson/bats,aerickson/bats,ev3dev/bats,ztombol/bats,Tiger66639/bats