Documented passing a local truststore to bin/plugin

docs/plugins/plugin-script.asciidoc

@@ -68,7 +68,7 @@ plugin from GitHub, run one of the following commands:
 [source,shell]
 -----------------------------------
 sudo bin/plugin install lmenezes/elasticsearch-kopf <1>
-sudo bin/plugin install lmenezes/elasticsearch-kopf/1.x <2>
+sudo bin/plugin install lmenezes/elasticsearch-kopf/2.x <2>
 -----------------------------------
 <1> Installs the latest version from GitHub.
 <2> Installs the 1.x version from GitHub.
@@ -104,6 +104,15 @@ For instance, to install a plugin from your local file system, you could run:
 sudo bin/plugin install file:///path/to/plugin.zip
 -----------------------------------
 
+The plugin script will refuse to talk to an HTTPS URL with an untrusted
+certificate. To use a self-signed HTTPS cert, you will need to add the CA cert
+to a local Java truststore and pass the location to the script as follows:
+
+[source,shell]
+-----------------------------------
+sudo bin/plugin -Djavax.net.ssl.trustStore=/path/to/trustStore.jks install https://....
+-----------------------------------
+
 [[listing-removing]]
 === Listing and Removing Installed Plugins