docs: Add contributing process

CONTRIBUTING.md

@@ -1,8 +1,61 @@
 ## Contributing Guide
 
-To Be Decided.
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 
-1. Fork this repo locally.
-2. Find or Open [GitHub Issues].
-3. Commit your code on a separate branch.
-4. Send your Pull Request.
+
+- [Find Similar Issues or Pull Requests](#find-similar-issues-or-pull-requests)
+- [Develop](#develop)
+- [Create Commit and Push](#create-commit-and-push)
+- [Open Pull Request](#open-pull-request)
+- [Update Local Repository](#update-local-repository)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+
+
+## Find Similar Issues or Pull Requests
+
+Nothing? Then, open a new Issue first, and discuss problems or proposals.
+
+
+
+## Develop
+
+```sh
+# (1) Fork this repository on GitHub.
+
+# (2) Clone your fork repository to your local.
+git clone https://github.com/<username>/hugo-theme-iris.git
+
+# (3) Start Hugo Server with Docker.
+make up
+```
+
+
+
+## Create Commit and Push
+
+```sh
+git checkout -b feat-something-option
+vim ./path/to/files
+git add ./path/to/files
+git commit -m "feat: Add something option"
+git push origin feat-something-option
+```
+
+
+
+## Open Pull Request
+
+On Github.
+
+
+
+## Update Local Repository
+
+```sh
+git remote add upstream https://github.com/peaceiris/hugo-theme-iris.git
+git checkout master
+git pull upstream master
+```