Add Known Issues section to README

Author: mattt

README.md

@@ -359,6 +359,21 @@ Follow these steps to release a new version of `swift-doc`:
 - [x] Create a new [release](https://github.com/SwiftDocOrg/swift-doc/releases)
       that corresponds to the new tag.
 
+## Known Issues
+
+- Xcode cannot run unit tests (<kbd>⌘</kbd><kbd>U</kbd>)
+  when opening the swift-doc package directly,
+  as opposed first to generating an Xcode project file with
+  `swift package generate-xcodeproj`.
+  (The reported error is:
+  `Library not loaded: @rpath/lib_InternalSwiftSyntaxParser.dylib`).
+  As a workaround,
+  you can [install the latest toolchain](https://swift.org/download/)
+  and enable it in "Xcode > Preferences > Components > Toolchains".
+  Alternatively,
+  you can run unit tests from the command line
+  with `swift test`.
+
 ## License
 
 MIT