Document amalgamated build on building page (#935)

Author: bnoordhuis

docs/docs/building.md

@@ -26,6 +26,16 @@ make
 This will build the `qjs` and `qjsc` executables and other test tools. Head over [here](./cli) for
 instructions on how to use them.
 
+## Amalgamated builds
+
+The amalgamated build rolls individual source files into a single big file.
+It's a good choice for projects that want to use QuickJS without CMake.
+
+Download quickjs-amalgam.zip from https://github.com/quickjs-ng/quickjs/releases
+
+To enable the std, os and bjson modules, compile quickjs-amalgam.c with
+`-DQJS_BUILD_LIBC`.
+
 ## Debug builds
 
 ```bash