Use `npm ci`, instead of `npm install` in the README

Signed-off-by: tpmccallum tim.mccallum@fermyon.com

Just aligns the README file with the actual contribution docs that were update last week.

Content must go through a pre-merge checklist.

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

[x] The title, template, and date are all set
[x] Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
[x] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep $'\r' | wc -l and expect 0 as a result)
[x] Has passed bart check
[x] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
[x] Headings are using Title Case
[x] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
[x] Have tested with npm run test and resolved all errors
[x] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.