debian/rules: add a link to pypa/setuptools_scm#549

We are using SETUPTOOLS_SCM_PRETEND_VERSION because the fallback_version
feature in setuptools_scm does not work when git commands return errors
(eg. when building from a tarball or git repo where no tags are present).

Add a link to the upstream issue adjacent to where
SETUPTOOLS_SCM_PRETEND_VERSION is specified.

Upstream will start providing fallback_version in the next
release so we should be able to remove the environment variable once
the upstream setuptools_scm issue has been resolved.

Signed-off-by: David Aguilar <davvid@gmail.com>

debian/rules

@@ -7,6 +7,7 @@
 
 # Override setuptools_scm's version detection.
 # setuptools_scm requires git tags to be present in the repository.
+# https://github.com/pypa/setuptools_scm/issues/549
 export SETUPTOOLS_SCM_PRETEND_VERSION=4.1.0
 
 # Disable inclusion of private libraries in non-standard locations.