New README.md file for rule index

rules/README.md

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+# Bazel Rules
+
+This directory contains examples of Bazel rules. For additional information,
+please refer to [the documentation](https://docs.bazel.build/versions/master/skylark/concepts.html).
+
+## Getting started
+
+These examples cover the most basic features: creating a rule, creating actions,
+passing information from a dependency to a target.
+
+* [empty](empty/): Minimal example to show the creation of a rule.
+
+* [attributes](attributes/): Example of a rule with attributes.
+
+* [actions.run](actions_run/): Example where a binary target is used as implicit
+  dependency and executed by the rule.
+
+* [actions.write](actions_write/): Example where a file is generated.
+
+* [expand_template](expand_template/): Example where a file is generated based
+  on a template.
+
+* [shell command](shell_command/): Example where a shell command is executed.
+
+* [default outputs](default_outputs/): Example of a rule with a declared output.
+
+* [depsets](depsets/): Example of a using a depset to gather transitive
+  information. Each target collects data from its dependencies.
+
+## Additional examples
+
+These examples explore more specific scenarios. They allow you to have more
+control on the behavior of the rules.
+
+* [executable](executable/): Example of an executable rule.
+
+* [test rule](test_rule/): Example of a test rule.
+
+* [computed dependencies](computed_dependencies/): Example with computed
+  dependencies. The set of implicit dependencies depends on the rule attributes.
+
+* [custom outputs](custom_outputs/): Example with `attr.output_list`.
+
+* [implicit output](implicit_output/): Example with implicit outputs. Some
+  outputs are available only when explicitly requested.
+
+* [aspect](aspect/): Example of using an aspect to collect information from
+  other rules.