update readme with engine info

Author: gajop

README.md

@@ -7,3 +7,5 @@ Make sure you have git configured to automatically fetch submodules:
 ```
 git config --global fetch.recurseSubmodules true
 ```
+
+Use engine from: https://springrts.com/dl/buildbot/default/ld33_prep/100.0.1-1-gffd50a7/