docs: documentation improvements

**Refactor and clarify the documentation in `README.md`**
- Add Highlights
- Add a Table of contents
- Clarify the way semantic-release works
- Clarify relationship with the CI environments
- Describe local install for Node projects (with a `package.json`) and global install for non-JavaScript projects
- Explain CI general configuration (environment variables and a run after all jobs are successful)
- Clarify configuration (via config file or CLI arguments)
- Clarify plugin roles and configuration
- Add doc for shareable configuration
- Add recipes
- Add resources (Videos, articles, tutorials)
- Add a Support section
- Add a Team section

**Add the following FAQs**
- How can I use a npm build script that requires the `package.json`’s version ?
- Can I use Semantic-release with Yarn?
- Can I use Semantic-release to publish non-JavaScript packages?
- Can I use Semantic-release with any CI service?
- Can I use Semantic-release with any GitLab?
- Can I use Semantic-release with any Git hosted environment?
- Can I skip the release to the npm registry?
- Can I use .npmrc options?
- How can I set the access level of the published npm package?
- Can I use Semantic-release to publish a package on Artifactory?
- Can I set the initial release version of my package to 0.0.1?
- Why does semantic-release require Node version >= 8?

**Clarify Nove 8 requirement and solutions**
- Add Node version requirement explanation and solutions
- [X] Display a link to the documentation when running on Node < 8 version

**Add recipes**
- Travis
- GitLab CI
- Travis with build stages - To be done in #573
- CircleCI workflows - To be done in #573

package.json

@@ -14,6 +14,10 @@
       "path": "cz-conventional-changelog"
     }
   },
+  "contributors": [
+    "Gregor Martynus (https://twitter.com/gr2m)",
+    "Pierre Vanduynslager (https://twitter.com/@pvdlg_)"
+  ],
   "dependencies": {
     "@semantic-release/commit-analyzer": "^5.0.0",
     "@semantic-release/error": "^2.1.0",
@@ -60,11 +64,11 @@
     "xo": "^0.18.2"
   },
   "engines": {
-    "node": ">=4",
-    "npm": ">=2"
+    "node": ">=8"
   },
   "files": [
     "bin",
+    "docs",
     "lib",
     "index.js",
     "cli.js"