Add an example log_config file

contrib/example_log_config.yaml

+# Example log_config file for synapse. To enable, point `log_config` to it in 
+# `homeserver.yaml`, and restart synapse.
+#
+# This configuration will produce similar results to the defaults within 
+# synapse, but can be edited to give more flexibility.
+
+version: 1
+
+formatters:
+  fmt:
+    format: '%(asctime)s - %(name)s - %(lineno)d - %(levelname)s - %(request)s- %(message)s'
+
+filters:
+  context:
+    (): synapse.util.logcontext.LoggingContextFilter
+    request: ""
+
+handlers:
+  # example output to console
+  console:
+    class: logging.StreamHandler
+    filters: [context]
+
+  # example output to file - to enable, edit 'root' config below.
+  file:
+    class: logging.handlers.RotatingFileHandler
+    formatter: fmt
+    filename: /var/log/synapse/homeserver.log
+    maxBytes: 100000000
+    backupCount: 3
+    filters: [context]
+
+
+root:
+    level: INFO
+    handlers: [console] # to use file handler instead, switch to [file]
+    
+loggers:
+    synapse:
+        level: INFO
+
+    synapse.storage:
+        level: INFO
+    
+    # example of enabling debugging for a component:
+    #
+    # synapse.federation.transport.server:
+    #    level: DEBUG
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