Add a definition of the term "Subprojects" that appears in Jenkins UI (#149)

See
jenkins-infra/jenkins.io#3948 (comment)
for the background on this pull request; in short, I saw "Subprojects"
in a Jenkins UI and didn't know what it meant. It would be nice if this
was defined in the README for the plugin that adds that UI element. :)

Author: atsalolikhin-spokeo

README.md

@@ -8,7 +8,9 @@ Parameterized Trigger Plugin
 This plugin lets you trigger new builds when your build has completed,
 with various ways of specifying parameters for the new build.
 
-  
+These new builds appear as "Subprojects" in the Jenkins UI when you
+are looking at a project that triggers them.
+
 You can add multiple configurations: each has a list of projects to trigger, a condition for when to trigger them 
 (based on the result of the current build), and a parameters section.