Readme: Minor changes (#1857)

Minor changes to the `README.md`. A few updated links and a few new sentences.

README.md

@@ -4,24 +4,24 @@
 
 Prism is a lightweight, robust, elegant syntax highlighting library. It's a spin-off project from [Dabblet](http://dabblet.com/).
 
-You can learn more on http://prismjs.com/.
+You can learn more on [prismjs.com](https://prismjs.com/).
 
-Why another syntax highlighter?: http://lea.verou.me/2012/07/introducing-prism-an-awesome-new-syntax-highlighter/#more-1841
+[Why another syntax highlighter?](https://lea.verou.me/2012/07/introducing-prism-an-awesome-new-syntax-highlighter/#more-1841)
 
-More themes for Prism: https://github.com/PrismJS/prism-themes
+[More themes for Prism!](https://github.com/PrismJS/prism-themes)
 
 ## Contribute to Prism!
 
 Prism depends on community contributions to expand and cover a wider array of use cases. If you like it, considering giving back by sending a pull request. Here are a few tips:
 
-- Read the [documentation](http://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. The minified files are also generated automatically.
+- 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. The minified files are also generated automatically.
 - The build system uses [gulp](https://github.com/gulpjs/gulp) to minify the files and build `prism.js`. Having gulp installed, you just need to run the command `gulp`.
 - 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 few huge PRs. If a PR includes changes I want to merge and changes I don't, handling it becomes difficult.
 - My time is very limited these days, so it might take a long time to review longer PRs (short ones are usually merged very quickly), especially those modifying the Prism Core. This doesn't mean your PR is rejected.
 - If you contribute a new language definition, you will be responsible for handling bug reports about that language definition.
-- If you add a new language definition, theme or plugin, you need to add it to `components.json` as well and rebuild Prism by running `gulp`, so that it becomes available to the download build page.
+- If you add a new language definition, theme or plugin, you need to add it to `components.json` as well and rebuild Prism by running `gulp`, so that it becomes available to the download build page. For new languages, please also add a few [tests](https://prismjs.com/test-suite.html) and an example in the `examples/` folder.
 
 Thank you so much for contributing!!