Add English doc for Save-log

site/src/site/sphinx/en/save-log.md

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+Log command outputs
+===================
+
+Log command outputs for later analysis, turned off by default.
+
+To turn it on, using [options](options.md) as:
+
+```sh
+$ options save-result true
+ NAME         BEFORE-VALUE  AFTER-VALUE
+----------------------------------------
+ save-result  false         true
+Affect(row-cnt:1) cost in 3 ms.
+```
+
+F.Y.I 
+
+1. logging file lies in: `{user.home}/logs/arthas-cache/result.log`;
+2. remember to clean up the file to save disk space.
+
+### Asynchronous log
+
+ :notes: :notes: 
+With the latest Arthas, you can log the outputs asynchronously in the background:
+
+```sh
+$ trace Test t >>  &
+job id  : 2
+cache location  : /Users/zhuyong/logs/arthas-cache/28198/2
+```
+
+F.Y.I
+
+1. `quit/exit` or `Ctrl+C` will not affect the asynchronous jobs :sparkles:; 
+2. default timeout for the background job is 1 day (you can set it via [options](options.md));
+3. outputs will be save to the `cache location` now (no matter what `save-result` is - not `~/logs/arthas-cache/result.log` any more).
+