bootstrap-haskell: mention the license

Since script is served separately from the main source code (can be opened over
https://get-ghcup.haskell.org) - mention the script license to the reciever.

bootstrap-haskell

@@ -8,6 +8,9 @@
 #   * BOOTSTRAP_HASKELL_GHC_VERSION
 #   * BOOTSTRAP_HASKELL_CABAL_VERSION
 
+# License: LGPL-3.0
+
+
 # safety subshell to avoid executing anything in case this script is not downloaded properly
 (