Improve usage docs to include full installation instructions

[getdave]

Currently docs are abit under developed.

• Add the composer config:

"require": {
    "getdave/wp-intervention": "1.1.0"
},
"repositories": [
    {
        "type": "vcs",
        "url": "https://github.com/getdave/wp-intervention"
    }
]

• Link to articles about WordPress and Composer.
• Add note about security plugins sometimes blocking requests to image URLs.