Add installation instructions

Author: mattt

README.md

@@ -11,27 +11,44 @@ for each class, structure, enumeration, and protocol
 as well as top-level type aliases, functions, and variables.
 
 For an example of generated documentation,
-[check out the Wiki for our fork of Alamofire][alamofire wiki]. 
+[check out the Wiki for our fork of Alamofire][alamofire wiki].
 
 > **Note**:
 > Output is currently limited to CommonMark,
 > but the plan is to support HTML and other formats as well.
 
-## Usage
+## Command-Line Utility
 
-`swift-doc` can be used from the command-line 
-or in a [GitHub Actions][github actions] workflow.
+`swift-doc` can be used from the command-line on macOS and Linux.
 
-### Command-Line Utility
+### Installation
 
-To run `swift-doc` from the command-line,
-clone the repository
-and do `swift run swift-doc` from within the project directory.
+#### Homebrew
+
+Run the following command to install using [Homebrew](https://brew.sh/):
 
 ```terminal
-$ git clone https://github.com/SwiftDocOrg/swift-doc.git
+$ brew install swiftdocorg/formulae/swift-doc
+```
+
+#### Manually
+
+Run the following commands to build and install manually:
+
+```terminal
+$ git clone https://github.com/SwiftDocOrg/swift-doc
 $ cd swift-doc
-$ swift run swift-doc path/to/SwiftProject/Sources --output Documentation
+$ make install
+```
+
+### Usage
+
+`swift-doc` takes one or more paths and enumerates them recursively,
+collecting all Swift files into a single "module"
+and generating documentation accordingly.
+
+```terminal
+$ swift doc path/to/SwiftProject/Sources --output Documentation
 $ tree Documentation
 $ Documentation/
 ├── Home
@@ -40,36 +57,33 @@ $ Documentation/
 └── _Sidebar.md
 ```
 
-`swift-doc` takes one or more paths and enumerates them recursively,
-collecting all Swift files into a single "module"
-and generating documentation accordingly.
 By default,
 output files are written to `.build/documentation`,
 but you can change that with the `--output` option flag.
 
-### GitHub Action
+## GitHub Action
 
 This repository also hosts a [GitHub Action][github actions]
 that you can incorporate into your project's workflow.
 
-The CommonMark files generated by `swift-doc` 
+The CommonMark files generated by `swift-doc`
 are formatted for publication to your project's [GitHub Wiki][github wiki],
-which you can do with 
+which you can do with
 [github-wiki-publish-action][github-wiki-publish-action].
 Alternatively,
 you could publish `swift-doc`-generated documentation to GitHub Pages,
 or bundle them into a release artifact.
 
-#### Inputs
+### Inputs
 
-- `inputs`: 
-  One or more paths to Swift files in your workspace. 
+- `inputs`:
+  One or more paths to Swift files in your workspace.
   (Default: `"./Sources"`)
 - `output`:
-  The path for generated output. 
+  The path for generated output.
   (Default: `"./.build/documentation"`)
 
-#### Example Workflow
+### Example Workflow
 
 ```yml
 name: Documentation
@@ -95,15 +109,15 @@ jobs:
           GITHUB_PERSONAL_ACCESS_TOKEN: ${{ secrets.GITHUB_PERSONAL_ACCESS_TOKEN }}
 ```
 
-* * *
+## Experimental Command-Line Tools
 
 In addition to `swift-doc`,
-this project currently ships with several, experimental executables
+this project currently ships with several, experimental tools
 that offer complementary functionality.
 It's unclear how all everything will ultimately fit together,
-but for now, 
+but for now,
 they're incubating in a shared monorepo
-(the intent is for each of them to eventually become 
+(the intent is for each of them to eventually become
 an option, subcommand, or plugin of `swift-doc`).
 
 * * *