Detect number of parameters, exit if less then 2.

Without this detection,

if a user gives no parameter, the error message will be:
> shift: can't shift that many

if a user gives no package name, the error message will be:
>  ! Cannot read property 'dependencies' of undefined

Tell the users to pass at least 2 parameters and then print `pnpm-install --help` to improve the UX.

bin/pnpm

@@ -1,4 +1,11 @@
 #!/usr/bin/env sh
+
+if [ "$#" -lt "2" ]; then
+  echo "You should pass at least 2 parameters to pnpm" 1>&2
+  pnpm-install --help
+  exit 1
+fi
+
 cmd=$1
 shift
 args=$*