Expose docker's root directory by default as part of docker info.

Signed-off-by: Vishnu kannan <vishnuk@google.com>

docs/reference/commandline/info.md

@@ -44,6 +44,7 @@ For example:
     Total Memory: 62.86 GiB
     Name: docker
     ID: I54V:OLXT:HVMM:TPKO:JPHQ:CQCD:JNLC:O3BZ:4ZVJ:43XJ:PFHZ:6N2S
+    Docker Root Dir: /var/lib/docker
     Debug mode (client): true
     Debug mode (server): true
      File Descriptors: 59

man/docker-info.1.md

@@ -51,6 +51,13 @@ Here is a sample output:
     Architecture: x86_64
     CPUs: 1
     Total Memory: 2 GiB
+    Name: docker
+    ID: I54V:OLXT:HVMM:TPKO:JPHQ:CQCD:JNLC:O3BZ:4ZVJ:43XJ:PFHZ:6N2S
+    Docker Root Dir: /var/lib/docker
+    Debug mode (client): false
+    Debug mode (server): false
+    Username: xyz
+    Registry: https://index.docker.io/v1/
 	
 # HISTORY
 April 2014, Originally compiled by William Henry (whenry at redhat dot com)