Add information for installing on FreeBSD
Add information for installing on FreeBSD

Added information for installing Elvish with `pkg` command (FreeBSD
package manager) and building Elvish from FreeBSD ports tree.

Signed-off-by: Adam Jimerson <0a1ce5db568b585e24ec79212f7ab30cdb820af8@gmail.com>

elves/elvish,elves/elvish,elves/elvish,elves/elvish,elves/elvish