Docs: Added minimal build instruction

Signed-off-by: Maxim Logaev <maxlogaev@proton.me>

README.md

@@ -1,2 +1,16 @@
 # kolibrios-sdk
 This is a C/C++ SDK for porting software to KolibriOS
+
+## Build
+At the project root:
+```sh
+source activate-env
+cmake -B build
+cmake --build build
+```
+
+## Testing
+Launch KolibriOS with the mounted `SDK_TOOLCHAIN_DIR` directory:
+```sh
+./run-kos.sh
+```