add strategy documentation

Author: ampuscas

src/main/resources/jenkins/branch/BranchSource/config.jelly

@@ -44,6 +44,7 @@
   <j:set var="descriptors" value="${descriptor.buildStrategiesDescriptors(it,sourceDescriptor)}"/>
   <j:if test="${!descriptors.isEmpty()}">
     <f:entry title="${%Build strategies}" field="buildStrategies">
+    <f:entry title="${%Strategy}" field="strategyId"></f:entry>
     <f:hetero-list name="buildStrategies" items="${instance.buildStrategies}"
                    descriptors="${descriptors}"
                    hasHeader="true" honorOrder="true"/>

src/main/resources/jenkins/branch/BranchSource/help-strategyId.html

@@ -0,0 +1,3 @@
+<div>
+    <b>Initial Index Build Prevention</b> modifies the indexing behavior so that any branches, pull requests, and tags discovered during the initial indexing create a Pipeline, but will not build it. The pipeline will remain inactive until an SCM event has been made to its corresponding branch/PR/tag.
+</div>