Add build instructions to README

Author: rjonaitis

readme.md

@@ -1 +1,17 @@
 LA9310 iqplayer based on https://github.com/nxp-qoriq/la931x_iqplayer
+
+# IQPlayer firmware for LimeSDR-Micro VSPA
+
+Building from source:
+
+```
+git clone github.com/myriadrf/LimeSDR-Micro_VSPA
+cd LimeSDR-Micro_VSPA
+cmake -B build && cd build
+make
+```
+
+Produced firmware images will be placed in LimeSDR-Micro_VSPA/build/Release directory.
+
+If "VSPA_TOOL" environment variable is available, VSPA toolchain will be used from that location.
+If "VSPA_TOOL" environment variable is not set, CMake will download VSPA toolchain and place it in project's artifacts directory.