Update README.md with Carthage installation

[alanzeino]

If you clone the repo today, you cannot just run make bootstrap after clone, because it runs bin/carthage.sh which assumes you already have carthage installed:

// Makefile
carthage-bootstrap:
	bin/carthage.sh bootstrap --platform iOS || exit 1

// bin/carthage.sh
carthage bootstrap --platform iOS

which results in:

❯ make bootstrap             
bin/carthage.sh bootstrap --platform iOS || exit 1
bin/carthage.sh: line 25: carthage: command not found
make: *** [carthage-bootstrap] Error 1

Obviously the bootstrap should install carthage, but I didn't want to do all that so I'm just updating the README to note this requirement.

Installing carthage is its own can of worms (how should it install, should it use homebrew, should it use a local pkg, should the pkg be bundled with the repo, what version of carthage, etc).

📲 What

A description of the change.

🤔 Why

Some background context on why the change is needed.

🛠 How

More in-depth discussion of the change or implementation.

👀 See

Trello, screenshots, external resources?

Before 🐛	After 🦋

♿️ Accessibility

• [ ] Tap targets use minimum of 44x44 pts dimensions
• [ ] Works with VoiceOver
• [ ] Supports Dynamic Type

🏎 Performance

• [ ] Optimized Blended Layers (screenshots)

✅ Acceptance criteria

• [ ] Steps to test this feature
• [ ] Environment to test on
• [ ] Issues to look out for

⏰ TODO

• [ ] Got planned refactors to come
• [ ] Will add more tests
• [ ] Need feedback on a design

[codecov[bot]]

Codecov Report

Merging #1736 (3d08d46) into main (6c367db) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1736   +/-   ##
=======================================
  Coverage   85.35%   85.36%           
=======================================
  Files        1270     1270           
  Lines      114842   114842           
  Branches    30335    30335           
=======================================
+ Hits        98028    98030    +2     
+ Misses      15762    15760    -2     
  Partials     1052     1052           

Impacted Files	Coverage Δ
Library/Navigation.swift	85.85% <0.00%> (+0.64%)	:arrow_up:

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[scottkicks]

Hey @alanzeino! Just FYI. The iOS team is actually working on migrating all Carthage dependencies over to Swift Package Manager. We're almost done in fact and will likely be removing bin/carthage.sh and the need to install Carthage altogether.