Add a Troubleshooting section to the README

Author: AngeloStavrow

README.md

@@ -235,6 +235,27 @@ Here's an excerpt of the graph generated for Alamofire:
 
 ![Excerpt of swift-doc-api Diagram for Alamofire](https://user-images.githubusercontent.com/7659/73189318-0db0e880-40d9-11ea-8895-341a75ce873c.png)
 
+## Troubleshooting
+
+Having trouble with SwiftDoc? Have a look at this section for help.
+
+### Homebrew: Failed to download resource "swift-doc"
+
+If using Homebrew to install or upgrade SwiftDoc fails with a `404` error similar to this:
+
+```bash
+curl: (22) The requested URL returned error: 404
+Error: Failed to download resource "swift-doc"
+```
+
+Try untapping and reinstalling SwiftDoc:
+
+```bash
+$ brew uninstall swift-doc
+$ brew untap swiftdocorg/formulae
+$ brew install swiftdocorg/formulae/swift-doc
+```
+
 ## GitHub Action
 
 This repository also hosts a [GitHub Action][github actions]