[readme] Mention swift-package-update workflow

Now that we build with HEAD, developers are more likely to need to run
swift-package-udpate themselves, so mention it in the build
instructions.

Author: benlangmuir

README.md

@@ -23,11 +23,14 @@ SourceKit-LSP is built using the [Swift Package Manager](https://github.com/appl
 For a standard debug build:
 
 ```sh
+$ swift package update
 $ swift build
 ```
 
 After building, the server will be located at `.build/debug/sourcekit-lsp`, or a similar path, if you passed any custom options to `swift build`. Editors will generally need to be provided with this path in order to run the newly built server - see [Editors](Editors) for more information about configuration.
 
+SourceKit-LSP is designed to build against the latest SwiftPM, so if you run into any issue make sure you have the most up-to-date dependencies by running `swift package update`.
+
 ### Building on Linux
 
 The C++ code in the index requires `libdispatch`, but unlike Swift code, it cannot find it automatically on Linux. You can work around this by adding a search path manually.