Re-add quickstart to the main documentation page on GH

I decided to have quickstart always up-to-date and always available
on the github homepage of the project.

E.g. the idea is to show how to boostrap it and get going even before
reading the doc.

README.md

@@ -15,15 +15,79 @@ historical archival.
 1. Elixir "~> 1.7"
 2. Works on Linux, Windows, OS X and BSD
 
-## Installation
 
-Add Crawly to you mix.exs file
-
-```elixir
-def deps do
-    [{:crawly, "~> 0.6.0"}]
-end
-```
+## Quickstart
+
+1. Add Crawly as a dependencies:
+   ```elixir
+   # mix.exs
+   defp deps do
+       [
+         {:crawly, "~> 0.6.0"}
+       ]
+   end
+   ```
+2. Fetch dependencies: `$ mix deps.get`
+3. Create a spider
+
+   ```elixir
+   # lib/crawly_example/esl_spider.ex
+   defmodule EslSpider do
+     @behaviour Crawly.Spider
+     alias Crawly.Utils
+
+     @impl Crawly.Spider
+     def base_url(), do: "https://www.erlang-solutions.com"
+
+     @impl Crawly.Spider
+     def init(), do: [start_urls: ["https://www.erlang-solutions.com/blog.html"]]
+
+     @impl Crawly.Spider
+     def parse_item(response) do
+       hrefs = response.body |> Floki.find("a.more") |> Floki.attribute("href")
+
+       requests =
+         Utils.build_absolute_urls(hrefs, base_url())
+         |> Utils.requests_from_urls()
+
+       title = response.body |> Floki.find("article.blog_post h1") |> Floki.text()
+
+       %{
+         :requests => requests,
+         :items => [%{title: title, url: response.request_url}]
+       }
+     end
+   end
+   ```
+
+4. Configure Crawly
+   - By default, Crawly does not require any configuration. But obviously you will need a configuration for fine tuning the crawls:
+   ```elixir
+   # in config.exs
+   config :crawly,
+     closespider_timeout: 10,
+     concurrent_requests_per_domain: 8,
+     follow_redirects: true,
+     closespider_itemcount: 1000,
+     output_format: "csv",
+     item: [:title, :url],
+     item_id: :title,
+     middlewares: [
+       Crawly.Middlewares.DomainFilter,
+       Crawly.Middlewares.UniqueRequest,
+       Crawly.Middlewares.UserAgent
+     ],
+     pipelines: [
+       Crawly.Pipelines.Validate,
+       Crawly.Pipelines.DuplicatesFilter,
+       Crawly.Pipelines.CSVEncoder,
+       Crawly.Pipelines.WriteToFile
+     ]
+   ```
+5. Start the Crawl:
+   - `$ iex -S mix`
+   - `iex(1)> Crawly.Engine.start_spider(EslSpider)`
+6. Results can be seen with: `$ cat /tmp/EslSpider.csv`
 
 ## Documentation