BUILDING.txt explains the way to import projects to eclipse. However, when you follow the instruction, you can't import the entire source tree (e.g., hadoop-project directory). In addition to this, maven-eclipse-plugin has already retired. There is a simpler way to import it without maven-eclipse-plugin.
Importing projects to eclipse
When you import the project to eclipse, install hadoop-maven-plugins at first.
$ cd hadoop-maven-plugins
$ mvn install
Then, generate eclipse project files.
$ mvn eclipse:eclipse -DskipTests
At last, import to eclipse by specifying the root directory of the project via
[File] > [Import] > [Existing Projects into Workspace].