create readme for wokringdir

sshpiperd/upstream/workingdir/README.md

@@ -0,0 +1,56 @@
+# Working Directory for SSHPiper
+
+`Working Dir` is a `/home`-like directory. 
+SSHPiperd read files from `workingdir/[username]/` to know upstream's configuration.
+
+e.g.
+
+```
+workingdir tree
+
+.
+├── github
+│   └── sshpiper_upstream
+└── linode
+    └── sshpiper_upstream
+```
+
+when `ssh sshpiper_host -l github`, 
+sshpiper reads `workingdir/github/sshpiper_upstream` and the connect to the upstream. 
+
+## User files
+
+*These file MUST NOT be accessible to group or other. (chmod og-rwx filename)*
+
+ * sshpiper_upstream
+
+    * line starts with `#` are treated as comment
+    * only the first not comment line will be parsed
+    * if no port was given, 22 will be used as default
+    * if `user@` was defined, username to upstream will be the mapped one
+
+```
+# comment
+[user@]upstream[:22]
+```
+    
+```
+e.g. 
+
+git@github.com
+
+google.com:12345
+
+```
+
+ * authorized_keys
+  
+   OpenSSH format `authorized_keys` (see `~/.ssh/authorized_keys`). Used for `publickey sign again(see below)`.
+
+ * id_rsa
+ 
+   RSA key for `publickey sign again(see below)`.
+   
+ * known_hosts
+ 
+   when `upstream-workingdir-stricthostkey` is set, upstream server's public key must present in known_hosts