Docs/file transfer protocols (jenkinsci#98)

* Update docs with valid file transfer protocols

According to the Groovy docs for fileTransfer protocols the valid
options are in lower case. Reference found here:
https://javadoc.io/doc/org.hidetake/groovy-ssh/latest/org/hidetake/groovy/ssh/session/transfer/FileTransferMethod.html

* Add another sshGet example

Some useful options for the sshGet commands lacked examples, e.g.,
filterRegex, and as such another example stage was added.

---------

Co-authored-by: Daniel Edholm <[email protected]>

README.adoc

@@ -91,7 +91,7 @@ Most of the steps in this plugin require a common step variable called `remote`,
 
 |fileTransfer
 |String
-|File transfer method, that is `SFTP` or `SCP`. Defaults to `SFTP`.
+|File transfer method, that is `sftp` or `scp`. Defaults to `sftp`.
 
 |encoding
 |String
@@ -373,12 +373,17 @@ node {
   def remote = [:]
   remote.name = 'test'
   remote.host = 'test.domain.com'
+  remote.fileTransfer = 'scp'
   remote.user = 'root'
   remote.password = 'password'
   remote.allowAnyHosts = true
   stage('Remote SSH') {
     sshGet remote: remote, from: 'abc.sh', into: 'abc_get.sh', override: true
   }
+  stage('Retrieve files with regex') {
+    def regexPattern = ".+\\.(log|csv)\$"
+    sshGet remote: remote, from: '/home/jenkins/', filterRegex: regexPattern, into: 'tests/', override: true
+  }
 }
 ```