added some readme

tikz/tikz-sizes/Readme.md

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+Compiled example
+----------------
+![Example](tikz-sizes.png)

tikz/udp-package-scheme/Readme.md

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+Compiled example
+----------------
+![Example](udp-package-scheme.png)

tikz/venn-diagramm/Readme.md

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+Compiled example
+----------------
+![Example](venn-diagramm.png)

tikz/vertex-cover-1/Readme.md

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+Compiled example
+----------------
+![Example](vertex-cover-1.png)

tikz/x-3-cubic-function/Readme.md

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+Compiled example
+----------------
+![Example](x-3-cubic-function.png)