Update docs how to seed db from fixtures

[faebebin]

If you try to run the app for first time locally on docker using docker-compose (or the according make commands) it throws errors on the server side because it needs some Menu object. Loading the existing fixture files solves the problem.

[codecov-commenter]

Codecov Report

Merging #252 (970e9aa) into master (ac26d15) will increase coverage by 36.43%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #252       +/-   ##
===========================================
+ Coverage   37.72%   74.15%   +36.43%     
===========================================
  Files          88      124       +36     
  Lines        3714     4705      +991     
===========================================
+ Hits         1401     3489     +2088     
+ Misses       2313     1216     -1097     

Impacted Files	Coverage Δ
qgis-app/plugins/templatetags/local_timezone.py	81.81% <0.00%> (ø)
qgis-app/api/views.py	96.00% <0.00%> (ø)
qgis-app/homepage.py	42.85% <0.00%> (ø)
qgis-app/middleware.py	70.00% <0.00%> (ø)
qgis-app/layerdefinitions/forms.py	100.00% <0.00%> (ø)
qgis-app/layerdefinitions/views.py	73.61% <0.00%> (ø)
qgis-app/lib/templatetags/sort_anchor.py	22.44% <0.00%> (ø)
qgis-app/wavefronts/views.py	77.21% <0.00%> (ø)
qgis-app/layerdefinitions/license.py	100.00% <0.00%> (ø)
qgis-app/custom_haystack_urls.py	44.44% <0.00%> (ø)
... and 59 more

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[dimasciput]

LGTM thanks