Add development documentation to cmd/pipectl/README.md

What would you like to be added:

Development documentation in cmd/pipectl/README.md. The file has been a one-line placeholder ("# pipectl: A Command Line Tool for PipeCD") since it was added in #4441, while other components such as cmd/piped and cmd/pipecd have full development guides in their READMEs.

It should cover the basics following the structure of cmd/piped/README.md: prerequisites, common make commands, and how to build and run pipectl locally during development. Usage documentation already exists on the docs site (user-guide/command-line-tool), so the README can link there for usage and focus on development.

Why is this needed:

CONTRIBUTING.md (Development section) links to this README and says "You can find detailed development information in the README file of each directory", but the file contains no development information. Contributors who follow that link find nothing about how to build or test pipectl.

Start by comparing cmd/pipectl/README.md with the development guide in cmd/piped/README.md, then check the Development section of CONTRIBUTING.md. Replace the placeholder with prerequisites, common make commands, and instructions for building and running pipectl locally; link to the existing user-guide/command-line-tool documentation for usage.