update readme

README.md

@@ -1,11 +1,15 @@
+[![Build](https://github.com/winitzki/sofp-solutions/workflows/build_and_test/badge.svg)](https://github.com/winitzki/sofp-solutions/actions/workflows/build-and-test.yml)
+
 # sofp-solutions
 
-Solutions for exercises in the book "Science of functional programming"
+Solutions for exercises in the book ["Science of functional programming"](https://github.com/winitzki/sofp)
 
 Solutions for each chapter is a submodule in this project.
 
 Common code may be put into the `common` submodule.
 
+Files under `worksheets/` may be arbitrary Scala code, not made into a test. SBT will ignore those files.
+
 # Code formatting
 
 The GitHub job will verify that all code is properly formatted.
@@ -14,4 +18,4 @@ To reformat code, run the script:
 
 `bash reformat_all_code.sh`
 
-Run this command before pushing any code changes. If formatting is not applied, builds will fail.
+Run this command before pushing any code changes in the test code. If formatting is not applied, builds will fail.