Added notes on issues with IntelliJ

Author: Dean Wampler

README.md

@@ -26,6 +26,7 @@
 | April 24, 2021    | Scala `3.0.0-RC3`  updates |
 | May 15, 2021      | Scala `3.0.0` final  updates. Almost done! |
 | May 22, 2021      | _Final_ updates for _Programming Scala, Third Edition_! |
+| July 24, 2021     | Notes on using IntelliJ |
 
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@@ -103,6 +104,23 @@ For other IDEs and text editors, try [Scala Metals](https://scalameta.org/metals
 
 After installing the required plugins, load this project in your IDE, which should detect and use the `sbt` project automatically. For eclipse, run the `sbt eclipse` task to generate project files, then import them.
 
+### Issues with IntelliJ
+
+One reader reported a problem when trying to run examples in IntelliJ: `scalac: Flag -encoding set repeatedly`. I could confirm this problem and I fixed it as follows:
+
+1. Open the preferences ("cmd-," on MacOS)
+2. Search for "scala"
+3. Select "Build, Execution, Deployment > Compiler > Scala Compiler"
+4. Select the "sbt" configuration in the list of Scala build configurations.
+5. Select "Additional compiler options:".
+6. Remove `-encoding utf-8` from the text, since it is already in the `build.sbt` file.
+
+After that, you should be able to select a type with a `main` and run it.
+
+The same reader also reported errors where multiple occurrences of the same name in a `@targetName` annotation collided. I believe this happens if you use `sbt` in a terminal to compile and then allow IntelliJ to do its own build. There are probably two copies of the class files on the resulting runtime classpath. For example, I saw this error when attempting to run `sbt console` in IntelliJ's `sbt shell`, but not when I used `sbt` in a terminal window.
+
+So, what worked for me was to only use the terminal to run `sbt clean`, then let IntelliJ build the software itself, but when I need to use `sbt console`, I use a terminal window.
+
 ### Using Scala Worksheets
 
 If you like working with _Scala worksheets_ in your IDE or editor, you may be able to load any of the REPL "script" files under `src/script` as a worksheet. If your environment is more restrictive, for example about the file extension used, then run the included `bash` script `./make-worksheets.sh` to copy all the REPL "script" examples to worksheet files. This command copies the tree `src/script` to `src/worksheet` and changes the `.scala` extension for all the files to `.worksheet.sc`, the VSCode convention. These behaviors are configurable. Use the `--help` option to see the details. If you are using Windows and you don't have `bash` available, e.g., through the Linux subsystem, then modify individual files as you see fit.