Update readme build instructions

Author: dopuskh3

README.md

@@ -19,6 +19,21 @@ There are two main APIs:
 The compress/decompress APIs mirror that of lz4, while the streaming API was
 designed to be a drop-in replacement for zlib.
 
+### Building against an external libzstd
+
+By default, zstd source code is vendored in this repository and the binding will be built with
+the vendored source code bundled.
+
+If you want to build this binding against an external static or shared libzstd library, you can
+use the `external_libzstd` build tag. This will look for the libzstd pkg-config file and extract
+build and linking parameters from that pkg-config file.
+
+Note that it requires at least libzstd 1.4.0.
+
+```bash
+go build -tags external_libzstd
+```
+
 ### Simple `Compress/Decompress`