dual boot - How to add a GRUB2 menu entry for booting multiple Linux OS?

I already had RHEL 7 on my HDD(I have only one) and now recently installed Ubuntu 14.04. I am unable to create Grub2 entry so that I can use both OS, currently I can only use Ubuntu.

The result for #fdisk -l is :

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00098227


 Device    Boot   Start       End         Blocks    Id  System
/dev/sda1   *   314574848   524290046   104857599+  83  Linux
/dev/sda2       524292094   976771071   226239489    f  W95 Ext'd (LBA)
/dev/sda5       524292096   874369023   175038464    7  HPFS/NTFS/exFAT
/dev/sda6       875397120   976771071    50686976   8e  Linux LVM 

Disk /dev/mapper/rhel-swap: 4043 MB, 4043309056 bytes
255 heads, 63 sectors/track, 491 cylinders, total 7897088 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/rhel-swap doesn't contain a valid partition table

Disk /dev/mapper/rhel-root: 47.9 GB, 47857008640 bytes
255 heads, 63 sectors/track, 5818 cylinders, total 93470720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/mapper/rhel-root doesn't contain a valid partition table