Adopt Semantic Versioning and add a changelog

[anderslindho]

This PR proposes adopting Semantic Versioning for future recCaster releases. It adds a CHANGELOG.md based on Keep a Changelog and establishes an Unreleased section for upcoming changes.

Historical release entries are intentionally deferred to the tag migration discussed in #3, where the corresponding recCaster contents can first be verified.

Closes #8

[shroffk]

@ralphlange does this match with other (hopefully most) epics modules?

[ralphlange]

Not that it matters (we're Phoebus and services!), but: it's in line, yes.
(I was not aware of Keep a Changelog - interesting.)

[anderslindho]

I'd push back a little bit on that yes, @ralphlange. Many EPICS projects use implicit patch 0s, which is not allowed per semver. Some also use disallowed separators (hyphens over periods), and I am not sure they all fully respect the semver significance of MAJOR.MINOR.PATCH. Plus EPICS base itself does not follow semver (the leading "protocol" digit). But it's close enough.

[ralphlange]

Many EPICS projects are much older than semver. Some old habits (like the stupid RX-Y notation) never die.

Newer projects try to follow, and it is seen as a valuable standard to follow within the Core devs. I keep my yes.