Instructions to help writing reference documentation

Whitespace fixes following 3/16 discussion with David T. on the insertion of images in tables. Addendum: Unless one has a specific reason in mind, it is generally not a good idea to have leading or trailing whitespace on images in a table. Bad form: | ![e1](img/eigen_01.png) |. Trailing and leading whitespace around the table marker |. Good form: |![e1](img/eigen_01.png)|. Note that the table intrinsically furnishes its own padding around text and images in table cells. Such excess white space has to go somewhere, and it may unceremoniously wrap images, causing misalignment, or lead to excessive margins around images. Some Rainy Day Sunday after Breakfast, I’ll write a G’MIC Markdown Cheat Sheet, but until then, read it first in git log.