Update --client-certificate option help

README.md

@@ -404,6 +404,9 @@ Alternatively, refer to the [developer instructions](#developer-instructions) fo
     -2, --twofactor TWOFACTOR            Two-factor authentication code
     -n, --netrc                          Use .netrc authentication data
     --video-password PASSWORD            Video password (vimeo, youku)
+    --client-certificate                 Path to a single certificate file in 
+                                         PEM format, used to authenticate to the
+                                         site (including private key)
 
 ## Adobe Pass Options:
     --ap-mso MSO                         Adobe Pass multiple-system operator (TV

youtube_dl/options.py

@@ -373,7 +373,7 @@ def parseOpts(overrideArguments=None):
     authentication.add_option(
         '--client-certificate',
         dest='clientcertificate', metavar='PATH',
-        help='Path to a single certificate file in PEM format, used to authenticate to the site')
+        help='Path to a single certificate file in PEM format, used to authenticate to the site (including private key)')
 
     adobe_pass = optparse.OptionGroup(parser, 'Adobe Pass Options')
     adobe_pass.add_option(