docs: add CONTRIBUTING.md

[MichaelTaylor3d]

Adds CONTRIBUTING.md — part of an ecosystem-wide sweep (16 repos).

This repo needs its own version because it does NOT follow the ecosystem's
standard flow: it tracks master (not main), has no branch protection on
master (verified via gh api repos/DIG-Network/DIG_Browser/branches/master/protection
-> 404 "Branch not protected"), uses regular merge commits rather than
squash-merge-only, has no commitlint/version-gate requirement in the general
sense, and cuts releases from pushed tags rather than from merges to master.

Derived from this repo's actual workflow files (build-arm.yml, build-x64.yml,
build-x86.yml, ensure-version-increment.yml, publish-release.yml,
reusable-build.yml) rather than assuming the standard pipeline:

• ensure-version-increment.yml is the only check that runs on every PR — it
  looks for package.json/Cargo.toml at the repo root, finds neither here,
  and passes vacuously.
• The three build-*.yml workflows only trigger on workflow_dispatch or a
  pushed tag (never on PR or push to master), and each runs a 24-stage
  chained Chromium build via reusable-build.yml.
• publish-release.yml matches a completed build run's commit back to a tag
  and publishes a GitHub release; x64 is required, x86/ARM64 are optional.

No version bump: the repo has no root package.json or Cargo.toml, so
ensure-version-increment.yml has nothing to enforce for this PR.

No closing keyword — this documents process, it doesn't fix a ticket.