Update .env.template with documentation

.env.template

@@ -1,11 +1,16 @@
-# User shoud set these variables.
+#!/bin/bash
+
+# User should set these variables.
+#
+# Optional environment variables which you opt not to set should be
+# deleted/commented out.
 #
-# Path to llvm-config binary. Only needed if LLVM's `llvm-config` and
+# OPTIONAL: Path to llvm-config binary. Only needed if LLVM's `llvm-config` and
 # `FileCheck` binaries are not in the PATH.
-export LLVM_CONFIG=
-# Path to Yosys binary. Only needed if the `yosys` binary is not available on
-# your path, or if you want to use a specific version of Yosys.
-export YOSYS=
+export LLVM_CONFIG="/path/to/llvm-config"
+# OPTIONAL: Path to Yosys binary. Only needed if the `yosys` binary is not
+# available on your path, or if you want to use a specific version of Yosys.
+export YOSYS="/path/to/yosys"
 
 
 # Do not modify below this line.