When generating .md files from the man pages for hosted support purposes, it is sometimes helpful to have local information included. Provide a mechanism to do this.
If present, these two files: doc/local-md.preamble doc/local-md.postscript will be embedded in the mkmd.sh generated index.md file.
https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=27954dde34fcb8c306fa4d80fdbb5950f0949e03