Adds Skylark Remote Repository documentation

Fixes #1043

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+---
+layout: documentation
+title: Skylark Repository Rules
+---
+# Repository Rules
+
+**Status: Experimental**. We may make breaking changes to the API, but we will
+  announce them and help you update your code.
+
+An [external repository](/docs/external.md) is a rule that can be used only
+in the `WORKSPACE` file and enable non-hermetic operation at the loading phase
+of Bazel. Each external repository rule creates its own workspace, with its
+own BUILD files and artifacts. They can be used to depend on third-party
+libraries (such as Maven packaged libraries) but also to generate BUILD files
+specific to the host Bazel is running on.
+
+## Repository Rule creation
+
+In a Skylark extension, use the
+[repository_rule](lib/globals.html#repository_rule) function to create a new
+repository rule and store it in a global variable.
+
+A custom repository rule can be used just like a native repository rule. It
+has a mandatory `name` attribute and every target present in its build files
+can be refered as `@<name>//package:target` where `<name>` is the value of the
+`name` attribute.
+
+The rule is loaded when you explictly build it, or if it is a dependency of
+the build. In this case, Bazel will execute its `implementation` function. This
+function describe how to creates the repository, its content and BUILD files.
+
+## Attributes
+
+An attribute is a rule argument, such as `url` or `sha256`. You must list
+the attributes and their types when you define a repository rule.
+
+```python
+local_repository = repository_rule(
+    implementation=_impl,
+    local=True,
+    attrs={"path": attr.string(mandatory=True)})
+```
+
+`name` attributes are implicitely defined for all `repository_rule`s.
+To access an attribute, use `repository_ctx.attr.<attribute_name>`.
+The name of a repository rule is accessible with `repository_ctx.name`.
+
+If an attribute name starts with `_` it is private and users cannot set it.
+
+## Implementation function
+
+Every repository rule requires an `implementation` function. It contains the
+actual logic of the rule and is executed strictly in the Loading Phase.
+The function has exactly one input parameter, `repository_ctx`, and should
+always returns `None`. The input parameter `repository_ctx` can be used to
+access attribute values, and non-hermetic functions (finding a binary,
+exuting a binary, creating a file in the repository or downloading a file
+from the Internet). See [the library](lib/repository_ctx.html) for more
+context. Example:
+
+```python
+def _impl(repository_ctx):
+  repository_ctx.symlink(repository_ctx.attr.path, "")
+
+local_repository = repository_rule(
+    implementation=_impl,
+    ...)
+```
+
+## Examples
+
+For now, we only have one full example of usage of the `repository_rule`:
+[C++ auto-configured toolchain](https://github.com/bazelbuild/bazel/blob/9116b3e99af2fd31d92c9bb7c37905a1675456c1/tools/cpp/cc_configure.bzl#L288).
+
+This example uses a Skylark repository rule to automatically create the
+C++ configuration files for Bazel by looking for the local C++ compiler, the
+environment and the flags the C++ compiler supports.
+