Sanitise README and tests to help other contributors

.gitignore

@@ -1,2 +1,4 @@
 .flattened-pom.xml
 target
+.idea
+*.iml

README.md

@@ -102,13 +102,13 @@ void execute_code_that_manipulates_system_properties(
     //code under test that reads properties (e.g. "some.property") or
     //modifies them.
   });
-  
+
   //Here the value of "some.property" is the same like before.
   //E.g. it is not set.
 }
 ```
 
-     
+
 ### System.out and System.err
 
 Command-line applications usually write to the console. If you write such
@@ -322,19 +322,19 @@ simply have a question about System Lambda.
 
 ## Development Guide
 
-System Lambda is build with [Maven](http://maven.apache.org/). If you
-want to contribute code than
+System Lambda is built with [Maven](http://maven.apache.org/) and must be
+compiled under JDK8. If you want to contribute code then
 
 * Please write a test for your change.
-* Ensure that you didn't break the build by running `mvnw test`.
+* Ensure that you didn't break the build by running `mvnw clean verify -Dgpg.skip`.
 * Fork the repo and create a pull request. (See [Understanding the GitHub Flow](https://guides.github.com/introduction/flow/index.html))
 
 The basic coding style is described in the
 [EditorConfig](http://editorconfig.org/) file `.editorconfig`.
 
 System Lambda supports [GitHub Actions](https://help.github.com/en/actions)
 (Linux) and [AppVeyor](http://www.appveyor.com/) (Windows) for continuous
-integration. Your pull request will be automatically build by both CI
+integration. Your pull request will be automatically built by both CI
 servers.
 
 

src/test/java/com/github/stefanbirkner/systemlambda/RestoreSystemErrChecks.java

@@ -7,7 +7,7 @@
 import static com.github.stefanbirkner.fishbowl.Fishbowl.ignoreException;
 import static org.assertj.core.api.Assertions.assertThat;
 
-class RestoreSystemErrChecks {
+abstract class RestoreSystemErrChecks {
 
 	private final MethodUnderTest methodUnderTest;
 

src/test/java/com/github/stefanbirkner/systemlambda/RestoreSystemOutChecks.java

@@ -7,7 +7,7 @@
 import static com.github.stefanbirkner.fishbowl.Fishbowl.ignoreException;
 import static org.assertj.core.api.Assertions.assertThat;
 
-class RestoreSystemOutChecks {
+abstract class RestoreSystemOutChecks {
 
 	private final MethodUnderTest methodUnderTest;