Do not require Registration field in Team Admin form

[ellerychan]

The Admin form to Add a Team requires a value for the Registration field, but no value should be required as the team is not registered initially. The problem was discovered while debugging with @jaron42.

The Admin form is validating the form fields and wants a value supplied. I fixed this by making the Registration field read-only.

Because this was a minor change and there is little activity on this repo, I went ahead and pushed it so @jaron42 could get the fix.

After merging these changes, remember to:

python manage.py makemigrations
python manage.py migrate

[ellerychan]

I was bad and pushed directly to hsdc2016. It turned out weird. The merge just shows changes to the ReadMe file, which came from @jaron42. However, my changes seem to be there, too.