doc: add developer guidelines
doc: add developer guidelines

It's not obvious how the TAP output is displayed and the meaning of the
exit value.

This documentation is meant to be for developers who want to add a new
validation test.

Signed-off-by: Alban Crequy <5d61c11e6df8ed8ffd2a3627a86b97e825765970@kinvolk.io>

opencontainers/ocitools,opencontainers/runtime-tools,opencontainers/runtime-tools,liangchenye/ocitools,opencontainers/ocitools,liangchenye/ocitools