Readme: Mention npm ci (#2899)

README.md

@@ -17,6 +17,7 @@ Prism depends on community contributions to expand and cover a wider array of us
 
 - Read the [documentation](https://prismjs.com/extending.html). Prism was designed to be extensible.
 - Do not edit `prism.js`, it’s just the version of Prism used by the Prism website and is built automatically. Limit your changes to the unminified files in the `components/` folder. `prism.js` and all minified files are also generated automatically by our build system.
+- Use `npm ci` to install Prism's dependencies. Do not use `npm install` because it will cause non-deterministic builds.
 - The build system uses [gulp](https://github.com/gulpjs/gulp) to minify the files and build `prism.js`. With all of Prism's dependencies installed, you just need to run the command `npm run build`.
 - Please follow the code conventions used in the files already. For example, I use [tabs for indentation and spaces for alignment](http://lea.verou.me/2012/01/why-tabs-are-clearly-superior/). Opening braces are on the same line, closing braces on their own line regardless of construct. There is a space before the opening brace. etc etc.
 - Please try to err towards more smaller PRs rather than a few huge PRs. If a PR includes changes that I want to merge and also changes that I don't, handling it becomes difficult.

docs/index.html

@@ -63,6 +63,7 @@ <h2>Contribute to Prism!</h2>
 <ul>
 <li>Read the <a href="https://prismjs.com/extending.html">documentation</a>. Prism was designed to be extensible.</li>
 <li>Do not edit <code>prism.js</code>, it’s just the version of Prism used by the Prism website and is built automatically. Limit your changes to the unminified files in the <code>components/</code> folder. <code>prism.js</code> and all minified files are also generated automatically by our build system.</li>
+<li>Use <code>npm ci</code> to install Prism's dependencies. Do not use <code>npm install</code> because it will cause non-deterministic builds.</li>
 <li>The build system uses <a href="https://github.com/gulpjs/gulp">gulp</a> to minify the files and build <code>prism.js</code>. With all of Prism's dependencies installed, you just need to run the command <code>npm run build</code>.</li>
 <li>Please follow the code conventions used in the files already. For example, I use <a href="http://lea.verou.me/2012/01/why-tabs-are-clearly-superior/">tabs for indentation and spaces for alignment</a>. Opening braces are on the same line, closing braces on their own line regardless of construct. There is a space before the opening brace. etc etc.</li>
 <li>Please try to err towards more smaller PRs rather than a few huge PRs. If a PR includes changes that I want to merge and also changes that I don't, handling it becomes difficult.</li>