Update README
Add plugin main file with required info
Create constants with plugin paths (directory and url)
Add plugin main class
Add plugin constructor
Add plugin version to default `version` property of the main class
Add admin menu item
Add admin submenu items dynamically
Remove buggy character
Merge branch 'feature/core' into develop

* feature/core:
  Remove buggy character
  Add admin submenu items dynamically
  Add admin menu item
  Add plugin version to default `version` property of the main class
  Add plugin constructor
  Add plugin main class
Add database version property to main plugin class
Remove PHP closing tag
Add install method

Creates/updates the database tables when a install or upgrade is
executed
Add insert method

Insert data initial data into the plugin database
Merge branch 'feature/database' into develop

* feature/database:
  Add insert method
  Add install method
  Remove PHP closing tag
  Add database version property to main plugin class
Add 'Categories' link to plugin submenu on the admin
Add `setup_views` method to load the view for each page on the admin menu
Add categories view
Fix comment
List all the categories on the database