workflow: add CONTRIBUTING.md (#1739)

* workflow: add CONTRIBUTING.md

* style: rephrase and clean existing content

* chore: add pull requests specs

* chore: add commit specs

.github/CONTRIBUTING.md

+## Development Setup
+
+This project uses a monorepo setup that requires using [Yarn](https://yarnpkg.com) because it relies on [Yarn workspaces](https://yarnpkg.com/blog/2017/08/02/introducing-workspaces/).
+
+``` sh
+# Install dependencies & compile TypeScript utilities.
+yarn bootstrap
+
+# Clean dependencies.
+yarn clean
+
+# Useful when creating new submodules.
+yarn boot
+
+# Serve the docs.
+yarn dev
+
+# Build the docs.
+yarn build
+
+# Execute all the test suites.
+yarn test
+```
+
+## Core packages
+
+- **docs**: Docs of VuePress (do not publish to npm).
+- **vuepress**: VuePress CLI.
+- **packages**
+  - `core`: containing the Node.js API, the Plugin API, the Theme API, the Client SPA, etc.
+  - `markdown`: internal Markdown compiler.
+  - `markdown-loader`: internal Markdown loader.
+  - `plugin-active-header-links`: a plugin for active sidebar heading links.
+  - `plugin-google-analytics`: Google Analytics integration.
+  - `plugin-last-updated`: implementation of "last updated" feature.
+  - `plugin-medium-zoom`: `medium-zoom` integration.
+  - `plugin-nprogress`: `nprogress` integration.
+  - `plugin-pwa`: PWA plugin.
+  - `plugin-search`: search plugin, providing the `SearchBox` component.
+  - `shared-utils`: TypeScript utilities.
+  - `test-utils`: test utilities.
+  - `theme-default`: default theme.
+  - `theme-vue`: a theme tweak from default theme, used for the official Vue project.
+
+## Core packages not in main project
+
+> Previously, for quick iteration, these projects were kept in ULIVZ's workspace. In the future, we may want to build an independent GitHub group.
+
+- [awesome-vuepress](https://github.com/ulivz/awesome-vuepress)
+- [@vuepress/plugin-blog](https://github.com/ulivz/vuepress-plugin-blog)
+- [@vuepress/theme-blog](https://github.com/ulivz/vuepress-theme-blog)
+
+## Workflow
+
+### Issue
+
+> TODO
+
+### Pull requests
+
+- Create a feature branch from the default branch (`master`) and merge back against that branch.
+- It's OK to have multiple small commits as you work on the PR - GitHub automatically squashes them before merging.
+- Make sure tests pass.
+- If adding a new feature:
+  - Add accompanying test case(s).
+  - Provide a convincing reason to add this feature. Ideally, you should open a suggestion issue first and have it approved before working on it.
+- If fixing bug:
+  - If you are resolving an open issue, add `(fix #xxxx)` (`#xxxx` being the issue ID) in your PR title for a better release log, e.g. `chore(feat): implement SSR (fix #1234)`.
+  - Provide a detailed description of the bug in the PR. Live demo preferred.
+  - Add appropriate test coverage if applicable.
+
+### Substantial Changes
+
+> RFC flow, TODO
+
+## Code Specification
+
+> TODO
+
+## Commit specification
+
+Commit messages should follow the [commit message convention](./COMMIT_CONVENTION.md) so that changelogs can be automatically generated.