What is the purpose of this pull request?

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (provide an overview)

I installed node-fetch 2.x on my project with Yarn 2 (aka Yarn berry). When I call require('node-fetch'), I get this warning message:

(node:22404) [MODULE_NOT_FOUND] Error: node-fetch tried to access encoding, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.

It seems that encoding package is optional and only used in body.textConverted() method (discussed in #412).

So I added the following to package.json of node-fetch:

"peerDependencies": { 
    "encoding": "*"
},
"peerDependenciesMeta": {
    "encoding": {
        "optional": true
    }
}

Which issue (if any) does this pull request address?

Is there anything you'd like reviewers to know?