[docs] how to test your own zig version

contrib/own_zig.md

@@ -0,0 +1,25 @@
+How to test a different version of zig
+--------------------------------------
+
+Assume you want to test an unreleased version of zig. Here's how:
+
+- build zig in `build/` directory, following the official instructions.
+- pack it to something that looks like a release using this script:
+
+```bash
+#!/bin/bash
+set -xeuo pipefail
+
+archversion="zig-linux-x86_64-$(git describe)"
+dst="$HOME/rel/$archversion/"
+rm -fr "$dst"
+mkdir -p "$dst"/docs
+cp zig "$dst"
+cp -r ../lib "$dst"
+tar -C "$HOME/rel" -cJf "$HOME/$archversion.tar.xz" .
+```
+
+- send it to jakstys.lt or your mirror.
+- point toolchain/defs.bzl to the new version.
+- run tests (probably locally, where zig was built).
+- running this on sr.ht is left as an exercise for further tests.