Versioning: stages v0.1, firmware SemVer, software CalVer

TEMPLATE.md Versioning section; notes in README, CHANGELOG and the firmware/software READMEs. Source: contrib/product-template in B4L/proplan.

firmware/README.md

@@ -5,3 +5,8 @@ example `firmware/main-mcu/`. Drivers shared by the targets go to
 `firmware/lib/`; third-party stacks (HAL, USB-PD middleware) are submodules in
 `external/`. IDE build folders (`Debug/`, `Release/`, `build/`) are not
 committed.
+
+Versions: SemVer per target, tagged `firmware/<target>/vMAJOR.MINOR.PATCH`
+(for example `firmware/main-mcu/v0.1.0`). Each target folder has its own
+`CHANGELOG.md`, and the firmware reports its version at run time. TEMPLATE.md
+says what counts as breaking and how pre-releases are named.