Update release notes.

ReleaseNotes/v6_0_0.md

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+**Release Notes for SmartGridToolbox v6.0.0**
+
+Version 6.0.0 of SmartGridToolbox is a major, breaking release. Significant changes have been made to core classes and functions. The documentation has also been completely revamped.
+
+**Release Notes for SmartGridToolbox v5.1.0**
+
+Various incremental changes and improvements, including minor documentation changes, improvements to transformers and tap changers, cpuTime function in Stopwatch class, update to JSON third party header library.
+
+**Release Notes for SmartGridToolbox v5.0.0**
+
+Version 5.0.0 is a major release, mainly due to changes to the core ComponentCollection and ComponentPtr classes. Various other changes have been made, including adding a TapChanger class, the ability of Heartbeat objects to drive updates in other slave objects, and significant changes to the experimental client/server code.
+
+**Release Notes for SmartGridToolbox v4.0.1**
+
+This minor patch adds some missing m4 files in examples/PvDemo.
+
+**Release Notes for SmartGridToolbox v4.0.0**
+
+Version 4.0.0 of SmartGridToolbox is a major, breaking release. The most significant changes and additions are listed below.
+
+**Updated Load Model**
+The largest breaking change is the updated load model. ZIP loads can now be modeled as phase-ground, phase-phase, or even an arbitrary combination of the two, in a flexible manner. Whereas, previously, the load components were specified as a vector containing the phase-ground components, they are now specified as upper triangular matrices, with the diagonal elements representing phase-ground components and the upper-off-diagonal elements representing phase-phase components. Furthermore, to more naturally and consistently represent all types of loads, load components now specify *consumptions* rather than injections.
+
+**New `ComponentCollection` Class**
+Another major change is the addition of the `ComponentCollection` class. As its name suggests, this is a special container for storing components and exposing them through class interfaces in a const-correct manner. It is used to store `Bus`, `Branch`, `GenAbc` and `ZipAbc` objects in `Network`s, and to store `SimComponent` and `TimeSeries` objects in `Simulation`s. It functions like a cross between a `std::map` and `std::vector`. A related `ComponentPtr<T>` template is provided, that will remain a valid pointer into the collection even if a component is replaced by a different component with the same key in the collection. This can make the process of building and modifying `Network`s or `Simulation`s simpler and less error-prone.
+
+**Islanding Support**
+Solvers can now correctly handle networks where part of the network becomes islanded / isolated, due to a fault, for example. Each island is solved separately, with unsupplied islands being handled in an appropriate manner.
+
+**`PowerFlowNrPolSolver` Temporarily Removed**
+Due to the addition of the new load model and islanding support, the polar Newton-Raphson solver has been temporarily removed. It is expected to be re-introduced in a minor update.
+
+**New `VvTransformer` Class**
+A new V-V (open-Delta) transformer class has been added.
+
+**Other**
+Various other bug-fixes and improvements have been implemented.