Add info on FB client pickle

README.md

@@ -10,6 +10,7 @@ Basically, Matrix becomes your everything chat client!
 Also, the Facebook Messenger backend might get you rate-limited on login
 attempts if you restart the bridge too often, which will lock you out of
 your Facebook account asking you to recover your account and change your password.
+See below for information on how to avoid this situation.
 
 Current features:
 
@@ -72,3 +73,24 @@ a dictionnary whose keys are the following:
 ### Storage configuration
 
 - `db_type` and `db_path`: the database backend and path to use (see the [Gorm documentation](http://gorm.io/docs/connecting_to_the_database.html))
+
+
+## Facebook Messenger alternative login procedure
+
+The default login proedure for the Messenger backend is to log in with your email and password each time
+Easybridge starts. Unfortunately, if Easybridge restarts too often, this will get you rate limited on login
+attempts quite rapidly. To bypass this issue, the Messenger backend provides the ability to login
+using a *client pickle*, i.e. a serialized client that contains all of the cookies obtained during a single
+initial login procedure, so that further logins are not required any longer.
+
+To generate your client pickle, run the following command:
+
+```
+./external/messenger.py create_client_pickle
+```
+
+This procedure will ask for your email and password and attempt to log you in to Facebook.
+If it succeeds, it will print several dozen lines of data looking like `eJyVVlt....X9cgyfgY7mJaK`.
+Then, when configuring the Messenger backend in Easybridge, *do not enter your password*.
+Instead, enter the obtained client pickle string in the appropriate field.
+