doc: added prerequisites to readme

Author: RockRockX2

README.md

@@ -58,6 +58,13 @@ The `default` feature set is `repl` and `sqlite`. With the `default` features, `
 
 ### From source
 
+#### Prerequisites
+
+Building BDK requires gcc. If you do not have this installed run: 
+```` shell
+sudo apt-get install build-essential
+````
+
 To install a dev version of `bdk-cli` from a local git repo with the `electrum` blockchain client enabled:
 
 ```shell