doc: link to @bazel_skylib//gazelle/bzl [skip ci] (#857)

extend.rst

@@ -14,8 +14,10 @@ Extending Gazelle
 .. _proto.Package: https://godoc.org/github.com/bazelbuild/bazel-gazelle/language/proto#Package
 .. _rules_r: https://github.com/grailbio/rules_r
 .. _rules_sass: https://github.com/bazelbuild/rules_sass
-.. _#75: https://github.com/bazelbuild/rules_sass/pull/75
 .. _bazel_rules_nodejs_contrib: https://github.com/ecosia/bazel_rules_nodejs_contrib#build-file-generation
+.. _bazel-skylib: https://github.com/bazelbuild/bazel-skylib
+.. _@bazel_skylib//gazelle/bzl: https://github.com/bazelbuild/bazel-skylib/tree/master/gazelle/bzl
+.. _#75: https://github.com/bazelbuild/rules_sass/pull/75
 .. _#803: https://github.com/bazelbuild/bazel-gazelle/issues/803
 
 .. role:: cmd(code)
@@ -35,7 +37,8 @@ To extend Gazelle, you must do three things:
 * Write a `go_library`_ with a function named ``NewLanguage`` that provides an
   implementation of the Language_ interface. This interface provides hooks for
   generating rules, parsing configuration directives, and resolving imports
-  to Bazel labels.
+  to Bazel labels. By convention, the library's package name should match
+  the language (for example, ``proto`` or ``bzl``).
 * Write a `gazelle_binary`_ rule. Include your library in the ``languages``
   list.
 * Write a `gazelle`_ rule that points to your ``gazelle_binary``. When you run
@@ -47,6 +50,8 @@ Supported languages
 
 Some extensions have been published by the community.
 
+* `bazel-skylib`_ has an extension for generating ``bzl_library`` rules.
+  See `@bazel_skylib//gazelle/bzl`_.
 * `rules_sass`_ has an extension for generating ``sass_library`` and
   ``sass_binary`` rules (currently pending in PR `#75`_).
 * `rules_r`_ has an extension for generating rules for R package builds and