Improve debug.sh script

- Move to the `bin` directly so that SOS works properly.
- Check for LLDB and PowerShell programs.
- Display help text.

debug.sh

@@ -1 +1,21 @@
-lldb-3.6 -o "plugin load ./bin/libsosplugin.so" -- ./bin/powershell $@
+#!/usr/bin/env bash
+
+hash lldb-3.6 2>/dev/null || { echo >&2 "No lldb-3.6, please run 'sudo apt-get install lldb-3.6'"; exit 1; }
+test -x bin/powershell || { echo >&2 "No bin/powershell, please run './build.sh'"; exit 1; }
+
+cat << EOF
+
+Launching LLDB with SOS plugin...
+Type 'run' or 'r' to start PowerShell.
+Press Ctrl-C to interrupt PowerShell and run LLDB commands.
+Type 'exit' when interrupted to leave LLDB.
+
+Visit https://git.io/v2Jhh for CoreCLR debugging help.
+
+Most useful commands are 'clrstack', 'clrthreads', and 'pe'.
+
+EOF
+
+pushd bin
+lldb-3.6 -o "plugin load libsosplugin.so" -- ./powershell $@
+popd