Added CodingStyle document

CodingStyle

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+
+
+Section 0: Notes on the existing codebase
+-----------------------------------------
+
+Not all parts of Yosys adhere to this coding styles for historical
+reasons. When adding code to existing parts of the system, adhere
+to this guide for the new code instead of trying to mimic to style
+of the surrounding code.
+
+
+
+Section 1: Formatting of code
+-----------------------------
+
+- Yosys code is using tabs for indentation. Tabs are 8 characters.
+
+- A continuation of a statement in the following line is indented by
+  two additional tabs.
+
+- Lines are as long as you want them to be. A good rule of thumb is
+  to break lines at about column 150.
+
+- Opening braces can be put on the same or next line as the statement
+  opening the block (if, switch, for, while, do). Put the opening brace
+  on its own line for larger blocks.
+
+- Otherwise stick to the Linux Kernel Coding Stlye:
+    https://www.kernel.org/doc/Documentation/CodingStyle
+
+
+Section 2: C++ Langugage
+------------------------
+
+Yosys is written in C++11. At the moment only constructs supported by
+gcc 4.6 is allowed in Yosys code. This will change in future releases.
+
+In general Yosys uses "int" instead of "size_t". To avoid compiler
+warnings for implicit type casts, always use "SIZE(foobar)" instead
+of "foobar.size()". (the macro SIZE() is defined by kernel/yosys.h)
+
+Use range-based for loops whenever applicable.
+