add documentation link in ReadMe.md

README.md

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 # PowerSAS.m
 
+[Link Documentation](https://powersasm.readthedocs.io/en/latest/index.html)
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 **PowerSAS.m** is a robust, efficient and scalable power grid analysis framework based on semi-analytical solutions (SAS) technology. The **PowerSAS.m** is the version for MATLAB/Octave users. It currently provides the following functionalities (more coming soon!):
 
 * **Steady-state analysis**, including power flow (PF), continuation power flow (CPF), contingency analysis.
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 ### Key features
 * **High numerical robustness.** Backed by the SAS approach, the PowerSAS tool provides much better convergence than the tools using traditional Newton-type algebraic equation solvers when solving algebraic equations (AE)/ordinary differential equations (ODE)/differential-algebraic equations(DAE).  
 * **Enhanced computational performance.** Due to the analytical nature, PowerSAS provides model-adaptive high-accuracy approximation, which brings significantly extended effective range and much larger steps for steady-state/dynamic analysis. PowerSAS has been used to solve large-scale system cases with 200,000+ buses.
-* **Customizable and extensible.** PowerSAS supports flexible customization of grid analysis scenarios, including complex event sequences in extended simulation term.  
+* **Customizable and extensible.** PowerSAS supports flexible customization of grid analysis scenarios, including complex event sequences in extended simulation term.  
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