Provide some notes on customizing Command Palette

[davidhcefx]

Description

I have read the Command Palette section. However, after I created a file Packages/User/Default.sublime-commands with the following contents, My Command simply didn't show up in the Command Palette:

[
    {
        "caption": "My Command",
        "command": "noop"
    },
]

After inspecting other packages and googling around, I found the fact that invalid commands won't show up in the command palette, as described by this forum post.

Suggestion

Adding a simple note "invalid commands won't show up in the command palette" to the document would help.