Provide migration guide

Documentation/MigrationGuide.md

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+## StencilSwiftKit 2.0 (SwiftGen 5.0) ##
+
+### For template writers:
+
+* We've removed our `join` array filter as Stencil provides it's own version that accepts a parameter. If you were using StencilSwiftKit's version, replace instances of:  
+```{{ myArray|join }}```  
+with:  
+```{{ myArray|join:", " }}```
+* We've refactored our `snakeToCamelCase` to accept arguments, thus removing the need for `snakeToCamelCaseNoPrefix`. If you were using the latter, replace instances of:  
+```{{ myValue|snakeToCamelCaseNoPrefix }}```  
+with:  
+```{{ myValue|snakeToCamelCase:true }}```
+
+### For developers:
+
+We've removed the following deprecated `typealias`es:
+
+* `FilterError`: use `Filters.Error` instead.
+* `ParametersError`: use `Parameters.Error` instead.
+* `NumFilters`: use `Filters.Numbers` instead.
+* `StringFilters`: use `Filters.Strings` instead.