add *.md files to makefile install

[gador]

Motivation

After the merge of #8084 the files in libstore now include the documentation *.md files. These files do not get copied by make, which in turn causes errors when linking those header files. (See e.g. zhaofengli/attic#56)

This came apparent after the update of nix stable on nixpkgs in NixOS/nixpkgs#233439 which causes a regression for me.

Context

This trivial change now includes the *.md files which fixes the issue. (Tested on current nixpkgs master). In my case (tested with attic, see issue linked above), I run into a different issue later on, but I guess it is unrelated. The follow up issue is

attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: expected expression
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'me'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields1'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields2'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: expected expression
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'me'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields1'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields2'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: expected expression
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'me'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields1'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/suggestions.hh:23:5: error: use of undeclared identifier 'fields2'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: expected expression
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: use of undeclared identifier 'me'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: use of undeclared identifier 'fields1'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: use of undeclared identifier 'fields2'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: expected expression
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: use of undeclared identifier 'me'
attic>   /nix/store/n4zhp46qjwmcd0az6pwqs5y6hg7x9ml1-nix-2.18.1-dev/include/nix/content-address.hh:35:5: error: use of undeclared identifier 'fields1

(EDIT:
Please ignore the version identifier of nix. I overwrote it to test my fork which included the fix)

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[thufschmitt]

I'm a bit meh on that one as these .md files don't really have any good reason for being included from header files. It actually seems that only uds-remote-store does it (I suspect just by accident), and the relevant bits could easily be moved to the corresponding .cc file

[edolstra]

Yeah the bug here is definitely including an .md file from a header. We don't want to install .md files.