add localization documentation

[cbuchwald]

Description:

This PR adds a documentation how to use Localization with Extensions

Please wait until Localization is officially released before completing this PR!

Success criteria:

• The documentation is comprehensive

How to test (steps):

1.Imagine you are a medium experienced user who created 1-2 extensions before. Would you be able to localize a simple extension after reading this documentation?

Additional considerations (describe):

• [ ] This PR impacts NLU
• [ ] This PR might have performance implications
• [ ] This PR changes an existing data model (and might affect existing legacy data)
  • Examples: adding, renaming, removing a field or changing the format or constraints of a field

Security implications (describe):

• [ ] Possible injection vector
• [ ] Authentication/Access controls touched
• [ ] Sensitive Data could be exposed
• [ ] XSS
• [ ] Logging/Monitoring touched
• [ ] Exchanges data with external systems
• [x] No security implications

[dshire]

I think you also need to add a link to the Localization readme from the main readme

[alexteusz]

@cbuchwald what is the current status of this?