Document xkcd.gp usage and limitations in README

[Copilot]

The README was too sparse for the included xkcd.gp example. This updates it to surface the script’s existing behavior, limitations, and font/output requirements in a more useful form.

• Repository overview

  • Replaced the one-line intro with a short description of the repo as a small gnuplot example for xkcd-style plots.

• Notes from xkcd.gp

  • Added a ## Notes section summarizing the script’s documented constraints and setup:
    • not a full xkcd rendering
    • straight axes
    • function jitter applied only on y
    • same approach can be used on both x and y for datapoints
    • a proper solution would likely require a dedicated gnuplot terminal
    • Humor Sans font usage
    • TTF source and PFA conversion requirement for EPS
    • included Humor-Sans.pfa file already used by the script

• Usage guidance

  • Added a ## How to use section describing the two wobble helpers and when to use each:
    • jiggle(x) for relative variation
    • jigglea(x) for fixed-size absolute variation
  • Called out how the bundled example uses them and noted that running the script produces xkcd.eps.

jigglea(x) = x+range*(2*(rand(0)-0.5)*0.005)
jiggle(x)  = x*(1+(2*rand(0)-0.5)*0.015)

plot  jiggle(dpsin(x)) ls 10 t '', \
      jiggle(dpsin(x)) ls 1 t 'Damped Sin',\
      jiggle(dpcos(x)) ls 10 t '', \
      jiggle(dpcos(x)) ls 2 t 'Damped Cos',\
      jigglea((x/15)**2) ls 10 t '',\
      jigglea((x/15)**2) ls 11 t ''