README.md: Add build instructions to README.

Author: flowergrass

README.md

@@ -9,3 +9,33 @@ of Unicorn, and allows the virtual microcontroller to run in the browser.  This
 then gives a full MicroPython port running simulated-bare-metal in a web browser.
 
 For a running demo please visit https://micropython.org/unicorn
+
+Build Instructions
+------------------
+
+```
+$ git submodule update --init
+```
+
+Firmware binaries can be customized in the unicorn directory.
+
+```
+$ cd unicorn
+$ make CONFIG=pyboard
+```
+
+The web page may be built using
+
+```
+$ cd www-emu
+$ make
+```
+
+In order to build without using gzip (For example when testing with `$ python -m http.server`)
+
+```
+$ cd www-emu
+$ make nogzip
+```
+
+There is a critical bug in unicorn-engine which is addressed [here](https://github.com/unicorn-engine/unicorn/pull/880). In order for full functionality apply the patch to a unicorn submodule within a unicorn.js repository and build normally.