Update new memory unit tests, documentation

plugins/system/MEM_README.md

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+## Telegraf Plugin: MEM
+
+#### Description
+
+The mem plugin collects memory metrics, defined as follows. For a more complete
+explanation of the difference between `used` and `actual_used` RAM, see
+[Linux ate my ram](http://www.linuxatemyram.com/).
+
+- **total**: total physical memory available
+- **actual_free**: the actual amount of available memory that can be given instantly
+to processes that request more memory in bytes; this is calculated by summing
+different memory values depending on the platform (e.g. free + buffers + cached on Linux)
+and it is supposed to be used to monitor actual memory usage in a cross platform fashion.
+- **actual_used**: inverse of actual_free, see above
+- **actual_used_percent**: the percentage usage calculated as (total - actual_used) / total * 100
+- **used**: memory used, calculated differently depending on the platform and
+designed for informational purposes only.
+- **free**: memory not being used at all (zeroed) that is readily available; note
+that this doesn't reflect the actual memory available (use 'available' instead).
+- **used_percent**: the percentage usage calculated as (total - used) / total * 100
+
+## Measurements:
+#### Raw Memory measurements:
+
+Meta:
+- units: bytes
+- tags: `nil`
+
+Measurement names:
+- mem_total
+- mem_actual_free
+- mem_actual_used
+- mem_used
+- mem_free
+
+#### Derived usage percentages:
+
+Meta:
+- units: percent (out of 100)
+- tags: `nil`
+
+Measurement names:
+- mem_used_percent
+- mem_actual_used_percent