Adding example for deffixtype.

Closes #239
Improving working-with-packages topic to disambiguate the confusion I had back in July.  Thanks to Sol for providing the clarification.

Fixes #204
Making the three required arguments of deffixtype
really required by adding a guard that their 'and' be non-nil.  I
thought about doing something fancier, but I didn't want to think
about what might happen if a call to er occurred.  Presumably these
all happen at the top-level, but I don't know.

Fixes #240.
Merge remote-tracking branch 'upstream/master'
Adding a convention suffix of '-limit' to best-practices
Adding an introduction to GL inside the Best
Practices topic area.
Merge pull request #246 from ragerdl/master

6 CLs