docs, redesign README.md — Quick Start, feature table, extract CONTRIBUTING.md

[steven-varga]

Problem

The README has two compounding issues:

staging README — wrong audience
After a good badge row, build matrix, and three-paragraph description, the document drops directly into 110 lines of contributor onboarding (issue naming, branch naming, commit format, rebase workflow, PR process). There is no Quick Start, no installation instructions, no code example, and no mention of any v1.12 features. A first-time evaluator has no reason to stay.

The migration-in-progress language ("H5CPP is being migrated…") is now stale — v1.12.3 shipped.

release README — museum exhibit
The canonical release branch still carries the original 2018 README: gcc 7.4.0, clang 6.0, download links to h5cpp.org, and a typo in the subtitle ("Paralell") that has been there since the first commit. This is what GitHub shows on the default branch landing page.

Scope

Extract

Move the entire "Migration and Contribution Workflow" section (issue naming → branch naming → commit format → rebase workflow → pull requests) to a new CONTRIBUTING.md. Keep a one-line pointer in the README.

Rewrite README structure

[badges]
[build matrix]

# H5CPP — [corrected subtitle, fix "Paralell" typo]

[description paragraph — keep current, trim bold overuse]

## Quick Start
[10-line write/read example — vector<float>, CMake find_package snippet]

## Requirements
- C++20 or later
- HDF5 ≥ 1.10.x (tested ceiling: 1.12.2)
- CMake ≥ 3.22

## Installation
- apt / brew / vcpkg one-liners
- from-source cmake steps

## Supported Types
[table: primitives, STL containers, linalg libs (Armadillo, Eigen, Blaze, Blitz++, uBLAS, IT++, dlib, std::valarray)]

## v1.12 Highlights
[brief bullets: valarray, iterator containers, float16, rank-7, complex, HDF5 1.12.2 ceiling, Windows MSVC]

## Documentation
[link to vargalabs.github.io/h5cpp]

## Contributing
[one line → CONTRIBUTING.md]

## License

Acceptance criteria

• CONTRIBUTING.md contains the full contribution workflow, verbatim
• README fits on one GitHub screen before the fold for a typical desktop
• Quick Start example compiles against v1.12.3 headers
• Subtitle typo fixed
• "migration in progress" language removed
• staging and release carry the same README after the PR merges

[steven-varga]

Work merged into staging.