doc: add contributing doc

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contributing
+
+We are really glad you're reading this, because we need volunteer developers to enhance this open source project. Pls feel free to submit issues, bugfix or new features. We really appreciate any contributing work you made to the project.
+
+How to contribute the project? In general, we follow the "fork-and-pull" [Git workflow](https://nvie.com/posts/a-successful-git-branching-model/). Two main branches with infinite lifetime exist: **master** and **devel**. It is recommended that you should follow the workflow if you want to submit a patch to the project.
+
+* S1. **Fork** the repo on Github.
+* S2. **Clone** the project to your own machine.
+* S3. **Checkout** to `devel` branch of the project on your own machine.
+* S4. **Commit** changes on your own branch.
+* S5. **Push** your work back up to your fork.
+* S6. Submit a **Pull Request** so that we can review your changes.
+
+NOTE: Be sure to merge the latest from "upstream" before making a pull request!
+
+# Coding conventions
+
+Basically, codeing style should be consistent in the whole project. We recommend to use the [Linux kernel coding style](https://www.kernel.org/doc/html/v4.10/process/coding-style.html#).
+
+NOTE: As for indentations, we use 4-char indents, not 8-char indents. This is different from Linux kernel coding style.

doc/tutorial.md

@@ -766,7 +766,7 @@ host$ ping www.iqiyi.com
 host$ curl www.iqiyi.com
 ```
 
-<a id='ipv6-support'/>
+<a id='ipv6_support'/>
 
 # IPv6 Support