Add 'pw doctor'.

Add 'pw doctor' command to check environment is correctly set up
for pigweed. For now contains minimal checks.

Change-Id: Icb132c89bb272b52c57dae0451b24966b48d4e90
Bug: 28
diff --git a/pw_doctor/BUILD.gn b/pw_doctor/BUILD.gn
new file mode 100644
index 0000000..9eed4a2
--- /dev/null
+++ b/pw_doctor/BUILD.gn
@@ -0,0 +1,21 @@
+# Copyright 2019 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("$dir_pw_docgen/docs.gni")
+
+pw_doc_group("docs") {
+  sources = [
+    "docs.rst",
+  ]
+}