docs: Add RELEASING.md

Author: georgeguimaraes

RELEASING.md

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+# Releasing HNSWLib
+
+## Prerequisites
+
+- Hex.pm account with publish access to `hnswlib`
+- All tests passing on main
+
+## Steps
+
+### 1. Update version for release
+
+Edit `mix.exs` and change `@version "X.Y.Z-dev"` to `"X.Y.Z"`.
+
+### 2. Commit and push
+
+```bash
+git add mix.exs
+git commit -m "Bump version to X.Y.Z"
+git push origin main
+```
+
+### 3. Wait for CI
+
+- Tests must pass
+- Precompile builds all binaries (triggered after tests pass)
+- Monitor at: https://github.com/elixir-nx/hnswlib/actions
+
+### 4. Create and push tag
+
+```bash
+git tag -a vX.Y.Z -m "Release vX.Y.Z"
+git push origin vX.Y.Z
+```
+
+### 5. Wait for release CI
+
+- Precompile workflow builds all platform binaries
+- Creates GitHub release with all `.tar.gz` and `.sha256` files
+- Monitor at: https://github.com/elixir-nx/hnswlib/actions
+
+### 6. Generate checksum.exs
+
+```bash
+mix elixir_make.checksum --all --ignore-unavailable
+```
+
+### 7. Commit checksum
+
+```bash
+git add checksum.exs
+git commit -m "Add checksum.exs for vX.Y.Z"
+git push origin main
+```
+
+### 8. Publish to Hex
+
+```bash
+git checkout vX.Y.Z
+mix deps.get
+mix hex.publish
+```
+
+### 9. Bump to next dev version
+
+```bash
+git checkout main
+# Edit mix.exs: change @version "X.Y.Z" to "X.Y.(Z+1)-dev"
+git add mix.exs
+git commit -m "Bump version to X.Y.(Z+1)-dev"
+git push origin main
+```
+
+## Summary
+
+```
+main (X.Y.Z-dev) → bump to X.Y.Z → push → CI passes → tag vX.Y.Z →
+CI builds release → checksum.exs → hex.publish → bump to X.Y.(Z+1)-dev
+```