Document that ~ in flutter path name leads to Can't find flutter on PATH

[samhedin]

Just had the issue when setting up on a new computer. User also had the issue here: #175

[cgrand]

I don't think it's specific to Flutter: ~ is not expanded in variables. It's just some globbing performed on literal unquoted arguments by the shell.

[samhedin]

My regular shell found flutter when ~ was part of the path in .bashrc, but cljd didn't.
I agree that it's probably better practice to use $HOME but even the flutter docs use ~ so copy pasting from there will lead to this error.