docs: Add README.md

[thumbrise]

PR #19: Add README.md

Type: Documentation only (single new file)

What it adds

A comprehensive README.md for the otelext library — a set of extensions for the
OpenTelemetry Go SDK focused on telemetry filtering and sampling.

Key sections in the README

1. Badges — CI status, Go Reference, Apache 2.0 license
2. Features — Filter registry, FilterBasedSampler, CompositeSampler
3. Installation — go get github.com/thumbrise/otelext (Go 1.25+, OTel SDK v1.42+)
4. Quick Start — Code examples for registering filters and composing samplers
5. API reference — signal.Filter interface (ShouldDrop, Key, Description),
   registry functions (RegisterFilter, RegisteredFilters, ClearFilters),
   and signal/trace types (FilterBasedSampler, CompositeSampler)
6. TODO roadmap — Planned work across docs, functionality, tests, CI/CD, and infra
7. Development — Prerequisites (Go 1.25+, Task, golangci-lint v2.4+) and commands
8. License — Apache 2.0, Copyright 2026 thumbrise

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.