fix: explain missing Brotli submodule during build

[manuelsampedro1]

This improves the failure mode for git checkouts that have not initialized the bundled Brotli C submodule.

A clean checkout without submodules currently fails deep inside cc/clang:

clang: error: no such file or directory: 'brotli/c/common/constants.c'

Changes:

• add an early build.rs check for the required Brotli C source
• emit an actionable message telling contributors to run git submodule update --init --recursive
• document the required submodule initialization in README Requirements

Validation:

• git submodule update --init --recursive
• cargo test

Bounty context: BitgesellOfficial/bitgesell#39

[MyTH-zyxeon]

Review assist for maintainers:

This looks like a focused build-diagnostics improvement for git-checkout users who have not initialized the bundled Brotli C submodule. I checked the patch scope: README adds the exact git submodule update --init --recursive step, and brotlic-sys/build.rs fails early when brotli/c/common/constants.c is missing before invoking the C compiler.

Suggested merge checks:

• Build once from a checkout with the submodule initialized and confirm the existing cc::Build path is unchanged.
• Build once from a checkout with the submodule intentionally absent and confirm the new panic is the first actionable error and includes the missing source path plus the submodule command.
• Confirm packaged/crates.io builds are not affected if the release artifact includes the Brotli C sources and does not rely on git submodule state.

I do not see dependency, runtime compression, wallet/key, or live-node behavior changes in this patch as written; the main acceptance criterion is clearer failure mode coverage for source checkouts missing Brotli C sources.