Add contributing guideline for public repository (#275)

* Add contributing guideline for public repository

Signed-off-by: Kenji Miyake <kenji.miyake@tier4.jp>

* Fix CONTRIBUTING.md

Signed-off-by: Kenji Miyake <kenji.miyake@tier4.jp>

* Apply review comment

Signed-off-by: Kenji Miyake <kenji.miyake@tier4.jp>

CONTRIBUTING.md

@@ -0,0 +1,7 @@
+# How to contribute to AutowareArchitectureProposal
+
+Thank you for your interest in improving the AutowareArchitectureProposal repository! As this repository is simply a mirror of a private repository, created to propose a new Autoware reference implementation to the [Autoware Foundation](https://www.autoware.org/), issues and pull requests are not accepted.
+
+If you are interested in developing Autoware, then consider contributing to the [Autoware.Auto project](https://gitlab.com/autowarefoundation/autoware.auto) instead. All Autoware development and user discussion takes place within that repository, and everyone is welcome to create issues and merge requests!
+
+Before making any contributions to the Autoware.Auto project, please make sure to review the [contributor's guide](https://autowarefoundation.gitlab.io/autoware.auto/AutowareAuto/contributors-guide.html), and please refrain from asking questions about the AutowareArchitectureProposal repository via ROS Answers or an Autoware.Auto issue.

README.md

@@ -2,7 +2,12 @@
 
 [![autoware](https://user-images.githubusercontent.com/8327598/69472442-cca50b00-0ded-11ea-9da0-9e2302aa1061.png)](https://youtu.be/kn2bIU_g0oY)
 
-AutowareArchitectureProposal is a repository to explore and establish the architecture design of Autoware, an autonomous driving software managed by Autoware Foundation.
+AutowareArchitectureProposal is a repository to explore and establish the architecture design of Autoware, an autonomous driving software managed by the Autoware Foundation.
+
+> Note:
+>
+> - As this repository is simply a mirror of a private repository, issues and pull requests are not accepted. Please see [CONTRIBUTING.md](./CONTRIBUTING.md) for more details.
+> - In the documentation, some links may appear broken. These links are pointing to internal Tier IV documents, so please ignore them.
 
 ## Documentation