Document steps & dependencies needed to build flatbuffers

[jkuszmaul]

I was recently bringing myself back up to being able to build the flatbuffers repo and realized that the bazel rules now require that swift be installed, in addition to any dependencies that we already had. I can't seem to find any documentation listing out what users need to have on their system before being able to build/test everything in the repo currently.

This is probably indirectly documented by what dependencies exist for the CI workers.

Nothing urgent, but with a larger and larger set of supported languages, platforms, etc. any depndencies that aren't automatically pulled in via the MODULES.bazel or the such should be documented.

[jkuszmaul]

Note: For swift, this includes the instructions at https://github.com/bazelbuild/rules_swift?tab=readme-ov-file#quick-setup which include at least:

• Install swift toolchain (which if on Linux is https://www.swift.org/install/linux/tarball/ , which involves finding the right tarball to download, extracting it, and placing it in your PATH).
• Configuring your default CC to be clang (could also be configured with appropriate toolchain configuration for the flatbuffers repo itself).