Outdated README Generation Instructions in CONTRIBUTING.md

[Adebayo120]

The instructions in CONTRIBUTING.md for regenerating the top-level README.md are outdated. They currently reference a Makefile and make command, but the actual process uses Go's go generate directive as defined in doc.go.

Details:
The doc.go file uses a go:generate directive to automate README generation:

//go:generate go run golang.org/x/example/internal/cmd/weave@latest -o ../../README.md ./README.src.md

The correct workflow is to edit README.src.md and then run:

go generate ./internal/readme

from the repository root, which regenerates the top-level README.md.
The current documentation suggests using make, but there is no Makefile present, leading to confusion for contributors.

Proposed Solution:
Update the "Updating the README" section in CONTRIBUTING.md to reflect the use of go generate instead of make. This will ensure contributors follow the correct process and avoid unnecessary errors.

Impact:
This change will improve the contributor experience and keep documentation accurate and up-to-date.