Add contributor hints and explanation of pending.dd

Author: wilzbach

changelog/pending.ddoc

@@ -3,7 +3,33 @@ $(DIVC version,
 $(P
 $(B $(LARGE $(LINK2 http://nightlies.dlang.org, Download D nightlies)))$(BR)
 $(SMALL $1)
+
 )
+
+$(HR)
+
+$(P
+This changelog has been $(B automatically generated) from all commits in $(B $(CODE master))
+since the last release.
+)
+$(P
+    $(UL
+        $(LI The full-text messages are assembled from the $(CODE changelog/) directories of the respective repositories:
+            $(LINK2 https://github.com/dlang/dmd/tree/master/changelog, $(CODE dmd)),
+            $(LINK2 https://github.com/dlang/druntime/tree/master/changelog, $(CODE druntime)),
+            $(LINK2 https://github.com/dlang/phobos/tree/master/changelog, $(CODE phobos)),
+            $(LINK2 https://github.com/dlang/tools/tree/master/changelog, $(CODE tools)),
+            $(LINK2 https://github.com/dlang/dlang.org/tree/master/changelog, $(CODE dlang.org)),
+            $(LINK2 https://github.com/dlang/installer/tree/master/changelog, $(CODE installer)), and
+            $(LINK2 https://github.com/dlang/dub/tree/master/changelog, $(CODE dub)).
+        )
+        $(LI See the $(LINK2 https://github.com/dlang-bots/dlang-bot#automated-references, DLang-Bot documentation) for details on referencing Bugzilla.
+        The DAutoTest PR preview doesn't include the Bugzilla changelog.)
+    )
+)
+
+$(HR)
+
 $3
 )
 CHANGELOG_SEP_HEADER_TEXT_NONEMPTY=$(BR)$(BIG $(RELATIVE_LINK2 bugfix-list, List of all upcoming bug fixes and enhancements in D $(VER).))