Update release notes for v0.2-pre4

Signed-off-by: Grant Likely <grant.likely@arm.com>

Author: glikely

source/revhistory.rst

@@ -8,13 +8,14 @@
    0.1       2016-MAY-24 Initial prerelease version. Imported |epapr| text
                          into reStructured Text format and removed Power ISA
                          specific elements.
-   0.2-pre3  2017-DEC-12 Third prerelease version.
+   0.2-pre4  2017-DEC-18 Fourth prerelease version.
 
                          * Added more recommended generic node names
                          * Added interrupts-extended
                          * Additional phy times
                          * Filled out detail in source language chapter
                          * Editorial changes
+                         * Added changebar version to release documents
 
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