[R6RS] new guiding principle (readability)

[William D Clinger]

I was asked to draft a new guiding principle (readability).
It goes under "Like R5RS Scheme, R6RS Scheme should:"

  * allow programmers to read each other's code, and
    allow development of portable programs that can
    be executed in any conforming implementation of
    Scheme;

There is obviously some overlap with the first guiding
principle listed under "In addition, R6RS Scheme should".
In my opinion, we should recognize that portable programs
could be and were developed in R5RS Scheme.

Will