Documented the release of CppSharp 0.8.21.

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

docs/releases/CppSharp-0.8.21-1.8.2018.md

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+# CppSharp 0.8.21 1.8.2018
+
+Only generated the system module in C# generators.
+Fixed missing save of generated module template to outputs.
+Fixed code generator to generate the system module even in single file per unit mode.
+Silenced verbose duplicate constructor and operator warnings.
+Improved the defaults of necessary generation options to improve accessibility.
+Called the parser setup code in CLI.
+Only included header files when scanning directories in CLI.