Update SOLUTIONS.md (skyzh#77)

A simple solution of Mini-LSM

SOLUTIONS.md

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 You can add your solution to this page once you finish any full week of the tutorial. You may have a one-sentence introduction of what you have done in your solution and any special functionalities you have implemented.
 
 ## Week 1
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+* [pj/mini-lsm-simple-solution](https://github.com/pjzhong/mini-lsm-solution): A simple solution of Mini-LSM.
 ## Week 2
 
 ## Week 3