Update README with build instructions.

README.md

@@ -1,9 +1,17 @@
+# MacRes - A cross-platform resampler
+
 (Fork of https://github.com/ohac/tn_fnds)
 
-MacRes - short for Mac Resampler
+MacRes is short for Mac Resampler
 
 This is an executable designed to be called by https://github.com/titinko/utsu (UTSU).
 Despite the name, it can be compiled for Windows, Mac, or Linux using the Makefile.
 
 The Windows/Linux versions of this resampler can also be used with the UTAU program:
 https://en.wikipedia.org/wiki/Utau.
+
+## Building from source
+
+This program uses two external libraries: [libpyin](https://github.com/Sleepwalking/libpyin) and [libgvps](https://github.com/Sleepwalking/libpyin).
+
+You will need to git clone both repositories, install their libraries using make install, and update the LIB_PREFIX field in frq0003gen's Makefile to match the directory where the .a and .h files were generated. You will also need to manually edit pyin.h to wrap its function definitions with "extern C" as described in [this bug](https://github.com/Sleepwalking/libpyin/issues/10).