Add manual installation guide

[kt3k]

There're many edge cases where install.sh doesn't work well with users environment (e.g. missing unzip, broken curl, the shell is not bash or zsh ), but also we don't want to make install.sh more complex to support those edge cases.

To address those edge cases, I suggest we should have manual installation guide, which instructs the users to install Deno completely manually (like with the sections 1. Download, 2. Move, 3. Configure $PATH and each section describes some alternatives.)