docs: add source-install note and youmd disambiguation to README

[brainsparker]

Why

Two small honesty fixes for the README:

1. Every command in the README starts with npx you-md, but the package is not on npm yet, so each one fails for a new user today.
2. There is an unrelated, actively maintained package named youmd on npm with a confusingly similar name and pitch. Anyone who typos the install or searches npm can end up in the wrong project.

What changed

• A running-from-source callout right after the hero section: clone, npm install and build, use node dist/cli.js until the first npm release. Easy to delete the day you-md is published.
• A name note stating this project is you-md by Brian Sparker and is unrelated to the youmd package on npm.

Validation

Docs only. The from-source instructions are the exact commands verified in a clean environment (build plus 145/145 tests).

[brainsparker]

Obsoleted by #30. The package is now published as @brainsparker/you-md, the README install commands use the scoped name, and #30 added a package-name note under the hero that covers the youmd disambiguation. The from-source callout would now be inaccurate. Closing.