add example of using a skylib build setting (bazelbuild#166)

Work towards bazelbuild#154

rules/WORKSPACE

@@ -30,4 +30,5 @@ http_archive(
     sha256 = "97e70364e9249702246c0e9444bccdc4b847bed1eb03c5a3ece4f83dfe6abc44",
 )
 load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")
-bazel_skylib_workspace()
+
+bazel_skylib_workspace()

rules/starlark_configurations/use_skylib_build_setting/BUILD

@@ -0,0 +1,17 @@
+# We don't actually need an external repo to define Starlark settings. But
+# Skylib provides convenience macros that reduce boilerplate, so we'll use that
+# here.
+#
+# See https://docs.bazel.build/versions/master/skylark/config.html and
+# https://github.com/bazelbuild/bazel-skylib) for more info.
+
+load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
+load(":defs.bzl", "dessert")
+
+# This can be set on the command line by setting 
+# //starlark_configurations/use_skylib_build_setting:flavor=<flavor>.
+# We declare here that its default value is "strawberry".
+string_flag(name = "flavor", build_setting_default = "strawberry")
+
+# A rule that depends on `:flavor`which means it can read its value.
+dessert(name = "ice-cream")

rules/starlark_configurations/use_skylib_build_setting/README.md

@@ -0,0 +1,9 @@
+This is an example of how to use and read the pre-defined build
+settings in the `bazel_skylib` repo.
+
+To test it out, cd into this directory and run the following:
+
+```
+$ bazel build :ice-cream // => "flavor: strawberry"
+$ bazel build :ice-cream --//starlark_configurations/use_skylib_build_settings:flavor=rocky-road => "flavor: rocky-road"
+```

rules/starlark_configurations/use_skylib_build_setting/defs.bzl

@@ -0,0 +1,17 @@
+# Load the provider of the pre-made settings defined in bazel_skylib
+load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")
+
+def _impl(ctx):
+    # Access the value of --//starlark_configurations/use_skylib_build_setting:flavor for the target.
+    print("flavor: " + ctx.attr._flavor[BuildSettingInfo].value)
+    return []
+
+dessert = rule(
+    implementation = _impl,
+    attrs = {
+        # Depend on the build setting so that we can access it in the rule implementation.
+        # Use a private attribute (one that is prefixed with "_") so that target writers
+        # can't override the value.
+        "_flavor": attr.label(default = ":flavor"),
+    },
+)