Add a "Tips" section to the manual

[stvdwtt]

Right now the manual is pretty descriptive -- this parameter does this, this parameter does that.

It would be beneficial to add another section that has a list of norms and tips for performance (and maybe readability). This could cover things like how to interact with tensors (#133) and a warning about the scaling implications of using too many subdivisions.

[stvdwtt]

From a conversation with Patrick Brewick: Currently the manual is good for telling someone how to tweak an existing application to suit their needs. However, it isn't as helpful for someone starting with an entirely new set of governing equations.

I think a "Tips" section of the manual has a role to play here, although this could also require a dedicated section.