Add Docs for Tuning Systems with NNI (#1715)

* Add Docs for Tuning Systems with NNI

* Updates based on comments

* updates

* updates

* updates

docs/en_US/CommunitySharings/TuningSystems.md

+# Automatically tune systems with NNI
+
+As computer systems and networking get increasingly complicated, optimizing them manually with explicit rules and heuristics becomes harder than ever before, sometimes impossible. Below are two examples of tuning systems with NNI. Anyone can easily tune their own systems by following them.
+
+* [Tuning RocksDB with NNI](../TrialExample/RocksdbExamples.md)
+* [Tuning parameters of SPTAG (Space Partition Tree And Graph) with NNI](SptagAutoTune.md)
+
+Please see [this paper](https://dl.acm.org/citation.cfm?id=3352031) for more details:
+
+Mike Liang, Chieh-Jan, et al. "The Case for Learning-and-System Co-design." ACM SIGOPS Operating Systems Review 53.1 (2019): 68-74.