Make distcheck in gh action

[Davknapp]

currently the execution of make distcheck fails (https://github.com/DLR-AMR/t8code/actions/runs/8660464369/job/23748458003)
because of:

 make[2]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[1]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[1]: Entering directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[1]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[1]: Entering directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make  dist-gzip am__post_remove_distdir='@:'
make[2]: Entering directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make  distdir-am
make[3]: Entering directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
(GIT_DIR=../../.git git log > .ChangeLog.tmp && \
         cat .ChangeLog.tmp | ../../build-aux/git2cl > ChangeLog) ; \
        rm -f .ChangeLog.tmp
make[3]: *** No rule to make target 'README.md', needed by 'README'.  Stop.
make[3]: *** Waiting for unfinished jobs....
fatal: not a git repository: '../../.git'
make[3]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[2]: *** [Makefile:8984: distdir] Error 2
make[2]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make[1]: *** [Makefile:9089: dist] Error 2
make[1]: Leaving directory '/__w/t8code/t8code/build/t8-2.0.0-alpha.31-8b94/_build/sub'
make: *** [Makefile:9101: distcheck] Error 1
Error: Process completed with exit code 2.

enabling the successfull execution of make distcheck will close this Issue

[cburstedde]

Using a file README works in p4est and sc, presumably because it is included by default in the distribution.
README.md may not be covered by any assumed rules. You may want to add it to the EXTRA_DIST variable.