Enhance Swift Numerics README.md (technical writing perspective)

[8bitmp3]

Hey @markuswntr!

I've looked at your Swift Numerics PR (ApproximateEquality): apple#150 and thought we could bundle a few changes together.

I've made a few improvement suggestions to the entire README based on tech writing experience - hope you like them:

• Small change: "may migrate in the second" -> "may migrate into the second"
• Refactor: "new uses are discovered" (passive voice) -> "more users start using Swift Numerics." (active voice)
• Reduce the use of "simply" if possible (it's not necessary but is recommended)
• Refactor + replace the idiom ("to pull in") for internalization/inclusiveness:

- Swift Numerics modules are fine-grained; if you need support for Complex
+ Swift Numerics modules are fine-grained. For example, if you need support for
+ Complex numbers ...
- ... import ComplexModule¹ without pulling in everything else in the library:
+ ... import ComplexModule¹ as a standalone module:
...
- as well.
...
[Example in code blocks]

• Refactor:

- // All swift-numerics API is now available
+ // The entire `swift-numerics` API is now available

• Replace the use of "we" with "the Swift Numerics project". For example:

- Because we intend to make it possible to adopt Swift Numerics modules in the
- standard library at some future point...
+ The Swift Numerics project is intended to be adopted in the standard library
+ in future. Therefore, it...

• Provide three steps (1... 2... 3...) for the Using Swift Numerics in your project section, since it's a sequential process - this helps with UX
• Small refactor: add "that is"

- Swift Numerics is a standalone library separate from the core Swift project. In
+ Swift Numerics is a standalone library that is separate from the core Swift project. In

• Enhance several HowTo sections under "Contributing..." into discoverable subheadings (e.g. "to propose a new module" -> "# How to propose a new module")
• Add a "### Forums" section for improved discoverability
• Add "About" to enhance explanations of the links:

1. About [`RealModule`](Sources/RealModule/README.md)
    - [`ApproximateEquality`](Sources/RealModule/ApproximateEquality.swift)
2. About [`ComplexModule`](Sources/ComplexModule/README.md)

• Reformat the Markdown text to wrap at 80 characters (it's a good practice, just like in code)
• Turn complex sentences -> smaller easier to follow sentences
• Use second person ("you") instead of first person ("I", "we"), if you can
• Add blank lines between the section titles and first paragraphs
• Add blank lines between before and after code blocks

Let me know what you think.

Cheers!

[markuswntr]

@8bitmp3 I am happy to merge your changes so we can discuss your suggestions within apple#150.
Thank you!