chore (doc): update readme partial template (#675)

add information about synchronicity flag

Author: minherz

.readme-partials.yaml

@@ -66,7 +66,28 @@ custom_content: |
       .build();
   logging.write(Collections.singleton(firstEntry));
   ```
-
+  
+  The library supports writing log entries synchronously and asynchronously.
+  In the synchronous mode each call to `write()` method results in a consequent call to Logging API to write a log entry.
+  In the asynchronous mode the call(s) to Logging API takes place asynchronously and few calls to `write()` method may be batched together to compose a single call to Logging API.
+  The default mode of writing is asynchronous.
+  It can be configured in the `java.util.logging` handler [configuration file](https://cloud.google.com/logging/docs/setup/java#javautillogging_configuration):
+  
+  ```
+  com.google.cloud.logging.LoggingHandler.synchronicity=SYNC
+  ```
+  
+  or in the code after initiating an instance of `Logging` by calling:
+  
+  ```java
+  logging.setWriteSynchronicity(Synchronicity.SYNC);
+  ```
+  
+  NOTE:
+  > Writing log entries asynchronously in some Google Cloud managed environments (e.g. Cloud Functions)
+  > may lead to unexpected results such as absense of expected log entries or abnormal program execution.
+  > To avoid these unexpected results, it is recommended to use synchronous mode.
+  
   #### Listing log entries
   
   With Logging you can also list log entries that have been previously written. Add the following