Replace GitBook with HonKit in Readme file

URLs pointing to GitBook now redirect to the current version of GitBook,
thus being unusable to check what was the intended link content
(e.g. https://legacy.gitbook.com/).

Also, I haven't been able to find a usable GitBook Editor link, but
instead needed to run `npm honkit serve` locally to visualize local
changes.

Closes DjangoGirls#1726

Author: adamantike

README.md

@@ -11,7 +11,7 @@ The Django Girls Tutorial is licensed under a [*Creative Commons Attribution-Sha
 
 The source code of the tutorial is [hosted on GitHub](https://github.com/DjangoGirls/tutorial). The GitHub [Fork & Pull workflow](https://help.github.com/articles/using-pull-requests) is used to accept and review changes.
 
-The tutorial uses the [GitBook](https://legacy.gitbook.com/) service for publishing its documentation. [See more information about how GitBook works](https://help.gitbook.com/).
+The tutorial uses the [HonKit](https://github.com/honkit/honkit) project for publishing its documentation. [See more information about how HonKit works](https://honkit.netlify.app/).
 
 The tutorial is written in [Markdown mark up language](https://help.github.com/articles/markdown-basics).
 
@@ -60,15 +60,24 @@ Either use the GitHub app for your operating system (mentioned above) or `git` c
 
     git clone git@github.com:yourgithubusername/tutorial.git
 
+Move to the folder containing the project, to run the following commands.
+
+    cd tutorial
+
 Then, create a branch for your new changes to sit in. It helps to call the branch something related to the changes you are going to make.
 
     git checkout -b contributing
 
-Download the [GitBook Editor](https://legacy.gitbook.com/editor) app to your computer.
+Install the project's requirements using [`npm`](https://docs.npmjs.com/cli/v8/configuring-npm/install).
+
+    npm install
 
-Then you can open the tutorial in GitBook Editor (*File* > *Open book*).
+To preview and serve local files, with auto-reload capabilities, run HonKit using:
 
-Make any changes in the tutorial using GitBook and then save changes (*Book* > *Save all*).
+    npx honkit serve
+
+The local server will be available at http://localhost:4000.
+If auto-reload is slow, you can temporarily remove unwanted languages from `LANGS.md`, to speed up the process.
 
 Then commit the changes using `git` and push the changes to your remote GitHub repository.
 
@@ -97,8 +106,6 @@ Example:
     To git@github.com:miohtama/tutorial.git
        b37ca59..fe36152  contributing -> contributing
 
-If you don't want to download the GitBook Editor app you can also go to the [GitBook website](https://legacy.gitbook.com/), sign up for free and work directly in your browser.
-
 # Making a pull request
 
 After you have finished your changes you need to create [a pull request](https://help.github.com/articles/using-pull-requests)  on GitHub. DjangoGirls will get notified about the pull request, review your changes, suggest any corrections if needed and then *pull* your changes to the master version.