docs: Add PACKAGING.md instructions (#1342)

Author: mati865

PACKAGING.md

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+# Recommendations for Packaging Wild linker
+
+## Binaries
+
+This repository consists of Wild linker and linker-diff binaries. We recommend providing the `wild`
+binary and symlink `ld.wild` pointing to it, as this enables `-fuse-ld=wild` usage with Clang.
+Linker-diff is mainly a tool to aid Wild development, so you most likely don't want to package it.
+
+## Building
+
+This project uses Cargo as its build system. The official releases are built with `--profile dist`
+that enables compiler's internal ThinLTO and strips the binaries. The benefit from ThinLTO
+is very mild in Wild's case, so it's up to you whether to use it. Musl releases also enable
+`--feature mimalloc`, see below for the explanation.
+
+### Optional features
+
+Wild has two optional build-time features:
+
+- `fork` (enabled by default) – an optimisation of process clean-up phase using `fork()`. Can be
+  disabled in the runtime via `--no-fork` flag.
+- `mimalloc` (disabled by default) – build and use Mimalloc as the allocator instead of the system
+  one. It performs marginally worse than Glibc in Wild's case, but much better than Musl.
+
+## Testing
+
+To test built binary, you can use `cargo test`, preferably with the same configuration as the build
+to avoid rebuilding. The testsuite is configured with `test-config.toml` file (default values are
+used if absent) and `WILD_TEST_IGNORE_FORMAT`.
+
+To tweak the configuration, you can copy `test-config.toml.example` to `test-config.toml` and edit
+it to your liking. You can learn more about the options
+at [Configuration file for tests](./CONTRIBUTING.md#configuration-file-for-tests). Just be careful
+with `run_all_diffs` option, it's meant mostly for Wild development and may cause false positives.
+
+Setting `WILD_TEST_IGNORE_FORMAT` disables format checks of C/C++ source files in tests which you
+might prefer because your `clang-format` may give different results.
+
+## Issues
+
+If you, or the users of your package, encounter any issues or pain points with Wild, don't hesitate
+to report them to us or reach out for help.