docs: document SOURCE_DATE_EPOCH in environment variables help

Add a short entry for $SOURCE_DATE_EPOCH to the env var table shown
in 'helm help'. The variable is honored by 'helm package', 'install',
'upgrade', 'dep build', and 'dep update' to produce reproducible chart
archives. Full documentation belongs in helm-www.

Signed-off-by: Lohit Kolluri <lohitkolluri@gmail.com>

pkg/cmd/root.go

@@ -84,6 +84,7 @@ Environment variables:
 | $HELM_QPS                          | set the Queries Per Second in cases where a high number of calls exceed the option for higher burst values |
 | $HELM_COLOR                        | set color output mode. Allowed values: never, always, auto (default: never)                                |
 | $NO_COLOR                          | set to any non-empty value to disable all colored output (overrides $HELM_COLOR)                           |
+| $SOURCE_DATE_EPOCH                 | set a Unix timestamp for reproducible chart archives                                                       |
 
 Helm stores cache, configuration, and data based on the following configuration order: