Add instructions to readme

Author: GabLeRoux

README.md

@@ -1,5 +1,22 @@
 # unity-sdk-downloader
 
+Download and extract `UnitySetup-Android-Support-for-Editor-*.exe`, decompile `UnityEditor.Android.Extensions.dll` and search for android JDK, NDK and SDK versions.
+
+## Requirements
+
+* Python 3
+* [`7z` from 7zip](https://www.7-zip.org/)
+* [`ilspycmd` from icsharpcode/ILSpy](https://github.com/icsharpcode/ILSpy)
+
+## Usage
+
+```bash
+python3 fetch.py
+python3 resolve.py
+```
+
+Results are stored in `versions.yml`
+
 ## License
 
 [MIT](LICENSE.md) © [Petteri Karppinen](https://github.com/pkarppi)