Improve oh-my-zsh install instructions to avoid compinit duplication …

[mkzer]

Summary

Improve the oh-my-zsh installation instructions in the README to avoid redundant compinit cache creation.

• The previous method caused compinit to run twice, which negatively impacted shell startup time
• This update replaces the outdated plugin-based usage with a recommended manual fpath declaration
• References issue oh-my-zsh installation instructions are sub-optimal #603 and proposes a clearer, optimized installation path

Fixes #603

• This compdef is not already available in zsh.
• This compdef is not already available in their original project.
• I am the original author, or I have authorization to submit this work.
• This is a finished work.
• It has a header containing authors, status and origin of the script.
• It has a license header or I accept that it will be licensed under the terms of the Zsh license.