add a note about custom headers not being used + ccache

[zingale]

in general, if you copy a header into a build directory, you need to clear the compiler cache or else the header will not be found.

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs