Added new README.md

doc/README.md

+## The documentation
+
+This folder contains different documentation:
+
+- UPBSlicer source code documentation (HTML and LaTeX)
+- UPBSlicer system manual
+- Several useful documents about Frama-C
+- Explanation of how to use UPBSlicer and Frama-C for example in the context of approximate computing
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