Documented the release of CppSharp 0.8.20.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

docs/releases/CppSharp-0.8.20-22.5.2018.md

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+# CppSharp 0.8.20 25.5.2018
+
+Fixed generation support for pointers to enums in C#.
+Fixed a case of functions converted to methods.
+Improve error handling in case of exceptions in CLI tool driver.
+Added an option to the CLI tool for enabling RTTI.
+Improved the  messages for errors in the CLI tool.
+Added parameter index to managed marshal variables.
+Changed the generated C# for const references to primitives as just primitives.
+Error messages are now written to stderr.