Documentation: describe about operators in metadata

Update documentation.

Fixes #1143

Author: wuqixuan

Documentation/unit-files-and-scheduling.md

@@ -179,6 +179,14 @@ app.service     282f949f.../10.10.20.1  active  running
 app.service     fd1d3e94.../10.0.0.1    active  running
 ```
 
+`MachineMetadata` also support relational operators, including `<=`, `>=`, `<`, `>`, `==` and `!=`:
+
+```
+[X-Fleet]
+MachineMetadata=ram<1024
+```
+This requires an eligible machine to have the `ram` less than 1024. The value must be numeral when using `<=`, `>=`, `<` or `>`.
+
 A machine is not automatically configured with metadata.
 A deployer may define machine metadata using the `metadata` [config option][config-option].