Add linear algebra design principles

[kgryte]

This PR

• adds a section on design principles for linear algebra APIs.

[rgommers]

Looks good, thanks @kgryte. Two other things come to mind:

1. Namespaces. NumPy has the weird split between in linalg functions between its main namespace and numpy.linalg. Other libraries tend to follow that. There's no optimal solution; we should say something about it here though.
2. You may want to include a part of Add specification for computing the dot product (linalg: vecdot) #137. A principle may be "orthogonality", which is a reason for including only certain "dot" functions.

[kgryte]

@rgommers I've added an orthogonality principle and a note regarding namespaces.

[kgryte]

I've removed the note, and will submit a follow-up PR regarding the mirroring of top-level linear algebra APIs in the linalg submodule as discussed during recent meetings.