Updated yarn add in README.md

[s0h311]

What?

updated the installation command for yarn.

Why?

For dev dependency it should be yarn add -D and not just yarn add

How?

Testing?

New Dependencies?

Screenshots

Suggested Reading?

Anything Else?

Checklist

• [x] I read CONTRIBUTING.md.
• [x] I read MAINTAINERSHIP.md#pull-requests.
• [ ] I added/updated unit tests for my changes.
• [ ] I added/updated integration tests for my changes.
• [ ] I ran the integration tests.
• [ ] I tested my changes in Node, Deno, and the browser.
• [ ] I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• [ ] I added/updated doc comments for any new/modified public APIs.
• [ ] My changes work for both new and existing PDF files.
• [x] I ran the linter on my changes.

[thm76]

I think this needs to be more nuanced because in some cases you will need to install pdf-lib as a dependency (I'm generating PDFs inside a React App) but in other cases it's a devDependency - I have another app where I'm generating PDFs at build time only.

In my opinion I would leave it as is, no "-D" flag. This is also consistent with the npm install command.

If the "-D" flag should be added then add it to npm install as well.