Remove the paragraph on different compilers

The link https://perl6.org/compilers/features is down, so removing this for now.

> Note that different backends implement slightly different sets of
> features. For a high-level overview of implemented and missing features,
> please visit [the features page on raku.org](https://perl6.org/compilers/features).

Add another paragraph

This is `Rakudo` compiler with `moar` and `jvm` backends. For historical compilers see https://www.raku.org/compilers/.

README.md

@@ -13,9 +13,10 @@ after more than just the bare compiler, please download [the latest
 Rakudo Star package](http://rakudo.org/downloads/star).
 
 Rakudo is currently the most developed implementation of the Raku
- language; though there have been other partial implementations in the
- past. [See the compilers page on the website for more](https://raku.org/compilers/).
-Rakudo has multiple backends: MoarVM and JVM. Note that each backend has a slightly different set of features.
+language; though there have been other partial implementations in the
+past. The `Rakudo` compiler has `moar`, `jvm` and `js` backends. Note
+that each backend has a slightly different set of features. For
+historical compilers see https://www.raku.org/compilers/.
 
 Recent changes and feature additions are documented in the `docs/ChangeLog`
 text file.