docs(readme): call out C toolchain prerequisite for source builds (#92)

Without gcc/clang on PATH, CGO is silently disabled and `go build` /
`go install` fails at the call site with `undefined: sqlitevec.LoadInto`
rather than a clear toolchain error, because `internal/sqlitevec/load.go`
is gated by `//go:build cgo`. The README's "Build from source" section
already assumed CGO_ENABLED=1 in the command but never said how to make
that actually work — leaving the next from-source builder to chase a
misleading "undefined symbol" error.

Add a short prerequisites paragraph above the build commands, pointing
at the existing comprehensive list in docs/getting-started.md.

Author: aksOps

README.md

@@ -177,6 +177,14 @@ embed or extract) return `503 {"code": "llm_disabled"}`.
 
 ## Build from source
 
+**Prerequisites:** Go ≥ 1.25, Node ≥ 22, and a working C toolchain
+(`build-essential` on Debian/Ubuntu, `xcode-select --install` on macOS,
+MinGW-w64 / MSYS2 on Windows). Without `gcc` on `PATH`, CGO is silently
+disabled and the build fails at the call site with a misleading
+`undefined: sqlitevec.LoadInto` rather than a clear toolchain error,
+because `internal/sqlitevec/load.go` is gated by `//go:build cgo`. Full
+list: [`docs/getting-started.md`](docs/getting-started.md#prerequisites).
+
 ```bash
 # First time on a connected machine
 npm --prefix ui ci                          # install UI deps