Goodbye guidelines (except GettingStarted)

Drop the parts that are being dropped.
Move the things that are being moved.
Also move the verilog stuff out of README and into the docs.
GettingStarted is less cut and dry, so hold off on that one.

docs/source/yosys_internals/extending_yosys/contributing.rst

@@ -0,0 +1,36 @@
+Contributing to Yosys
+=====================
+
+Coding Style
+------------
+
+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, especially blocks that contains
+  blank lines.
+
+- Otherwise stick to the Linux Kernel Coding Style:
+    https://www.kernel.org/doc/Documentation/process/coding-style.rst
+
+
+C++ Language
+~~~~~~~~~~~~
+
+Yosys is written in C++17.
+
+In general Yosys uses "int" instead of "size_t". To avoid compiler
+warnings for implicit type casts, always use "GetSize(foobar)" instead
+of "foobar.size()". (GetSize() is defined in kernel/yosys.h)
+
+Use range-based for loops whenever applicable.