Add note about development instructions in package developer docs

[tennox]

See
#906

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes # (issue)

Environment report

Oh My Fish version:   7
OS type:              Linux
Fish version:         fish, version 3.1.0
Git version:          git version 2.26.0
Git core.autocrlf:    no
Checking for a sane environment...
Your shell is ready to swim.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing tests pass locally with my changes
• I have updated the SHA256 checksum for the install script

[A1exKH]

LGTM.