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