docs: Clarify model commit message summary
docs: Clarify model commit message summary

Add a paragraph about prefixing commit message summaries with the
package/subsystem that they're changing to
code_contribution_guidelines.md.

stevenroose/btcd,aakselrod/btcd,davecgh/btcd,tb00/btcd,Stash-Crypto/btcd,stevenroose/btcd,wallclockbuilder/btcd,btcsuite/btcd,Roasbeef/btcd,adiabat/btcd,dajohi/btcd,dajohi/btcd,davecgh/btcd,ltcsuite/ltcd,ppcsuite/ppcd,dajohi/btcd,aakselrod/btcd,jongillham/btcd,ppcsuite/ppcd,ltcsuite/ltcd,wallclockbuilder/btcd,cqswxj2013/btcd,Roasbeef/btcd,jongillham/btcd,btcsuite/btcd,adiabat/btcd,tb00/btcd,Stash-Crypto/btcd,cqswxj2013/btcd