Add contributing/troubleshooting guide for devs

[pmengelbert]

For example, add the following to contributing.md

For future reference here are the stages of producing a mariner2/container target, and the intermediate targets representing to llb.State at various times:

1. Prepare the sources # debug/sources and mariner2/rpm/sources
2. Prepare the build container + the RPM spec # mariner2/rpm/spec
3. Prepare the buildroot # mariner2/rpm/buildroot
4. Run the build by invoking rpmbuild tool # mariner2/rpm
5. Prapare the final container + install the RPM from 3 # mariner2/container

for example to see what the tarred-up sources look like:

docker build -f path/to/dalec/spec.yml \
    --target=mariner2/rpm/sources \
    --output=type=local,dest=/tmp/rpmsources \
    .