docs: clarify public source build path vs private build.sh entrypoint

[ketpatil77]

What I noticed

The README has a public "Building from Source" path:

git clone https://github.com/altic-dev/FluidVoice.git
cd FluidVoice
open Fluid.xcodeproj

It also documents a no-signing build:

xcodebuild -project Fluid.xcodeproj -scheme Fluid -destination 'platform=macOS' build CODE_SIGNING_ALLOWED=NO

But the repository also includes build.sh, whose public/OSS branch currently exits with:

Public/OSS builds are not available from this repo entrypoint.
Use the private Fluid Intelligence build: sh build_with_FI_incremental.sh

Suggested docs change

Add a short note near "Building from Source" explaining which command path public contributors should use:

• use Xcode / xcodebuild for public OSS builds, and
• do not use ./build.sh unless they have the private Fluid Intelligence build setup.

Why

New contributors may try ./build.sh first because it is a top-level build script, then think source builds are unavailable. A one-line note would make the public build path clearer without changing code.