Update contribution instructions with fontforge --script

[Tehnix]

Description

Following issue #225 it seems fontforge has updated how it behaves after installing. For a consistent experience across
various operating systems (Ubuntu, macOS, etc), it seems we get better results by pre-pending fontforge --script ..
before running the font-patcher script.

Requirements / Checklist

• Read the Contributing Guidelines
• Read or at least glanced at the FAQ
• Read or at least glanced at the Wiki
• Scripts execute without error (if necessary):
  • If any of the scripts were modified they have been tested and execute without error, e.g.:
    • ./font-patcher Inconsolata.otf --fontawesome --octicons --pomicons
    • ./gotta-patch-em-all-font-patcher\!.sh Hermit
• Extended the README and documentation if necessary, e.g. You added a new font please update the table

What does this Pull Request (PR) do?

Updates the Contributing.md docs.

How should this be manually tested?

Verify that e.g. fontforge --script ./font-patcher src/unpatched-fonts/Hasklig/Regular/Hasklig-Regular.otf --complete also works locally for you.

If not, we should include both with and without.

Any background context you can provide?

While trying to follow the contribution instructions, the invocation of font-patcher keeps failing with

$ ./font-patcher src/unpatched-fonts/Hasklig/Regular/Hasklig-Regular.otf --complete
Nerd Fonts: FontForge module is probably not installed. [See: http://designwithfontforge.com/en-US/Installing_Fontforge.html]

Running the following instead works:

$ fontforge --script ./font-patcher src/unpatched-fonts/Hasklig/Regular/Hasklig-Regular.otf --complete

Check out #225 for more context.

What are the relevant tickets (if any)?

#225

Screenshots (if appropriate or helpful)

N/A

[ryanoasis]

thank you