docs: Version release plan (#28)

## Summary

Starting staging area for packaging newer ANSI C SOFA releases + pure
Julia impl

Contacted SOFA board to request tarbarlls

Motivation: Maintain a historical reference for users that is accessible
via easy jll builds

## 2019 - 2023 Release summary

| SOFA date     | What changed | New routines |
|---------------|---|---|
| 2019-07-22 | (starting point — what `SOFA_jll` 2019.7.22+0 ships) | —
|
| 2020-07-21    | Bug fixes only | none |
| 2021-01-25    | Bug and accuracy fixes | none |
| 2021-01-25_a  | Small bug-fix patch on top of 2021-01-25 | none |
| 2021-05-12 | Bug fixes + 3 new routines | `iauAtcc13`, `iauAtccq`,
`iauMoon98` |
| 2023-10-11 | Bug fixes only (STARPV/PVSTAR machine-precision tweaks,
`IYV` bumped to 2023, comment corrections in
A2TF/ATOIQ/C2T06A/FK45Z/FK54Z/HFK5Z/LTPB, `t_atccq` test fix) | none |

The only release with new API surface in this span is **2021-05-12**,
which adds three routines. All other intermediate releases are bug fixes
only, with no signature changes.

Sources:
- [ERFA README.rst — "Differences from
SOFA"](https://github.com/liberfa/erfa/blob/master/README.rst)
- [ERFA commit b2039ecd "Updating to SOFA
20210512"](liberfa/erfa@b2039ecd)
- [SOFA changes page (2023-10-11 release
notes)](https://www.iausofa.org/changes)

Author: icweaver

README.md

@@ -19,12 +19,18 @@ prove compliance and reproducibility of the core SOFA C functionality.
 
 ## Package Version <-> SOFA Release Correspondence
 
-The package versions correspond to the following releases of the SOFA C Library:
-
-| Package Version | SOFA Release |
-| --------------- | ------------ |
-| v1.0            | 2019-07-22   |
-| v0.1            | 2018-01-30   |
+The planned package versions below correspond to the following releases of the SOFA C Library:
+
+| Package Version | SOFA Release | build      |
+| :-------------- | :----------- | :--------  |
+| v2.0            | 2023-10-11   | pure Julia |
+| v1.5            | 2023-10-11   | jll        |
+| v1.4            | 2021-05-12   | jll        |
+| v1.3            | 2021-01-25_a | jll        |
+| v1.2            | 2020-07-21   | jll        |
+| v1.1            | 2019-07-22   | jll        |
+| v1.0            | 2019-07-22   | manual     |
+| v0.1            | 2018-01-30   | manual     |
 
 ## Notes