README/keys: Add 'TOGGLE_MUTE_TOPIC' hotkey to 'mute/unmute' topics.

New section 'Topic list actions' created in README.

New 'topic_list' category created in keys for mapping keys
belonging to topic list actions.

Author: sumanthvrao

README.md

@@ -198,6 +198,11 @@ source ~/.zshenv
 | Show/hide stream information & modify settings        | <kbd>i</kbd>                                  |
 | Show/hide stream members (from stream information)    | <kbd>m</kbd>                                  |
 
+### Topic list actions
+| Command                                               | Key Combination                               |
+|------------------------------------------------------ | --------------------------------------------- |
+| Mute/unmute Topics                                    | <kbd>M</kbd>                                  |
+
 ### Composing a message
 | Command                                                   | Key Combination                               |
 | -----------------------------------------------------     | --------------------------------------------- |

zulipterminal/config/keys.py

@@ -209,6 +209,11 @@
         'help_text': 'Mute/unmute Streams',
         'key_category': 'stream_list',
     }),
+    ('TOGGLE_MUTE_TOPIC', {
+        'keys': ['M'],
+        'help_text': 'Mute/unmute Topics',
+        'key_category': 'topic_list',
+    }),
     ('ENTER', {
         'keys': ['enter'],
         'help_text': 'Perform current action',
@@ -320,6 +325,7 @@
     ('searching', 'Searching'),
     ('msg_actions', 'Actions for the selected message'),
     ('stream_list', 'Stream list actions'),
+    ('topic_list', 'Topic list actions'),
     ('msg_compose', 'Composing a Message'),
 ])