readme: Delete trailing whitespace

In the `readme.md` document, delete the trailing whitespace that was
introduced in vultraz’s rewrite of the document.
readme: Fix typos

In the `readme.md` document, correct some typos that were introduced in
vultraz’s rewrite of the document.

Specifically, the corrections are as follows:

  - “different” (used as an adverb) → “differently”
  - “made used of”                  → “made use of”
  - “Battle for wesnoth”            → “Battle for Wesnoth”
  - “soD”                           → “SoD”
readme: Set text width to 80 columns and re-wrap

In the `readme.md` document, add a Vim modeline specifying a text width
of 80 columns (a figure decided upon by vultraz, to match shadowm’s
style), and re-wrap the text of the document to that width, after the
text width of the document became inconsistent in vultraz’s rewrite of
the document.