Add README marking this repo as a release mirror

[jsokol]

Description

Adds a README documenting that this repository is a read-only publish mirror of the SimpleRisk core:

• Each release arrives as a single tagged "SimpleRisk <version> Release" commit; tags track where a version currently stands (a re-issued version moves its tag).
• Direct commits and pull requests here can't land — branch protection restricts pushes to the release-sync App, and any direct change would be overwritten by the next sync.
• Points bug reports, feature requests, and contributions at support.simplerisk.com, and downloads at simplerisk.com/download.

Companion to the README added to simplerisk-extras; the equivalent sync workflow for this repo is simplerisk/code-development#1764.

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