Update Emmet Support Instructions for ~H sigil

[miXwui]

Instructions for using emmet with ~H sigils

I had to also add this line:

"emmet.includeLanguages": {
...
  "elixir": "html"
},

to get Emmet abbreviations to work inside ~H sigils.

Addition to https://github.com/phoenixframework/vscode-phoenix/issues/8 and https://github.com/phoenixframework/vscode-phoenix/pull/10

EDIT: Actually, not sure if this belongs here since it's ~H sigils are in Elixir .ex files :man_shrugging: