document shared memory segment issue
After discussion with @SamirTalwar-DA, we agree the CI script to clear
memory segments is a bit too dangerous to make it easy to run on
developer machines. Still, developers may run into similar issues if
they run lots of tests and/or do not reboot their laptop frequently.
On developer laptops, we usually spawn one PostgreSQL instance per
build/test that needs it (as opposed to CI where we create a single one
for the entire build; see `build.sh`), so they can actually build up
fairly quickly in some scenarios.
As an alternative, I have added a section to the README to cover what to
do if that issue happens.
CHANGELOG_BEGIN
CHANGELOG_END
