Prepare README.md files describing the new repositories structure

[MaciejBaj]

Parent #3022

Expected behavior

Write README files and provide the overview and explanation of the new Lisk repositories and their structure:
- [ ] Create README for custom transactions #3164 moved to 2.1.0

• Create README for the application-level folder Create README for the application-level folder #3163
• Update README for the framework-level folder Create README for the Lisk SDK #3162
• Create README for framework Create README for framework #3161
• Update README for the framework controller Update README for the framework controller #3156
• Update README for the framework modules Update README for the framework modules #3157
• Update README for the framework components Update README for the framework components #3158

Which version(s) does this affect? (Environment, OS, etc...)

1.6.0