document build directory

build/README.md

@@ -0,0 +1,11 @@
+This directory contains Dockerfiles and shell scripts
+for building Tek in a container. For now, only the
+GLIBC build works, as the Musl static build is unable
+to `dlopen` the system's `libjack.so`.
+
+Invoke from repo root, like this: `build/release-glibc.sh`.
+This will first build a Docker image, `tek:glibc`, which
+will contain all build-time dependencies; then, it
+will invoke a `cargo build --release` in a container
+spawned from that image, ultimately placing the
+release build in this directory, as `build/tek`.