Add README and .drawio file (apache#54)

README.md

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+
+This is the [website](https://pegasus.apache.org/) codebase of the Apache [Pegasus](https://github.com/apache/incubator-pegasus) project.
+
+It is a static website based on Jekyll, and almost all the documents are written in Markdown.
+
+If you found any mistake on the website, or you've developed a new feature for the Pegasus project, or you want to improve the UI/UE of the website, you are encouraged to update the website.
+
+# How to contribute
+
+## How to preview the website
+
+Install Jekyll at first https://jekyllrb.com/docs/
+
+```bash
+$ git clone [email protected]:apache/incubator-pegasus-website.git
+$ cd incubator-pegasus-website
+$ bundle exec jekyll serve
+...
+ Auto-regeneration: enabled for '.../incubator-pegasus-website'
+    Server address: http://127.0.0.1:4000
+  Server running... press ctrl-c to stop.
+```
+
+Then open your browser and visit http://127.0.0.1:4000
+
+## Submit your changes
+
+1. Update the website codebase, and preview it locally to check it act as expected.
+
+> If you want to update or draw a new diagram, you can use [draw.io](https://app.diagrams.net/) and open [assets/drawio/apache_pegasus_website.drawio](./assets/drawio/apache_pegasus_website.drawio) to draw it, and save it as a `.png` file, then put it into the `assets/images` directory.
+> Remember to submit the changed `.drawio` file together with the `.png` file.
+
+2. Submit the patch as a [pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests)
+
+3. Wait the project committers to review and merge your patch.
+
+## View on the website
+
+Open your browser and visit https://pegasus.apache.org/, you will see the result of your patch.
+
+Enjoy it!