Added Spring design doc

Description

@EgorkaKulikov, please review the Markdown-formatted design document.

The PR introduces no code changes. Manual scenario: the text itself has been previewed in the IntelliJ IDEA viewer.

[x] I've set the proper labels for my PR (at least, for category and component).
[X] PR title and description are clear and intelligible.
[ ] I've added enough comments to my code, particularly in hard-to-understand areas.
[ ] The functionality I've repaired, changed or added is covered with automated tests.
[ ] Manual tests have been provided optionally.
[ ] The documentation for the functionality I've been working on is up-to-date.