HADOOP-12019. update BUILDING.txt to include python for 'mvn site' in windows (Contributed by Vinayakumar B)

BUILDING.txt

@@ -282,6 +282,7 @@ Requirements:
 * Internet connection for first build (to fetch all Maven and Hadoop dependencies)
 * Unix command-line tools from GnuWin32: sh, mkdir, rm, cp, tar, gzip. These
   tools must be present on your PATH.
+* Python ( for generation of docs using 'mvn site')
 
 Unix command-line tools are also included with the Windows Git package which
 can be downloaded from http://git-scm.com/download/win.

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -817,6 +817,9 @@ Release 2.8.0 - UNRELEASED
     HADOOP-12018. smart-apply-patch.sh fails if the patch edits CR+LF files
     and is created by 'git diff --no-prefix'. (Kengo Seki via aajisaka)
 
+    HADOOP-12019. update BUILDING.txt to include python for 'mvn site'
+    in windows (vinayakumarb)
+
 Release 2.7.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES