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.

docs/source/yosys_internals/extending_yosys/index.rst

@@ -11,5 +11,6 @@ of interest for developers looking to customise Yosys builds.
    extensions
    build_verific
    functional_ir
+   contributing
    test_suites
 

docs/source/yosys_internals/extending_yosys/test_suites.rst

@@ -1,7 +1,7 @@
 Testing Yosys
 =============
 
-.. todo:: more about the included test suite
+.. TODO:: more about the included test suite and how to add tests
 
 Automatic testing
 -----------------
@@ -23,3 +23,76 @@ For up to date information, including OS versions, refer to `the git actions
 page`_.
 
 .. _the git actions page: https://github.com/YosysHQ/yosys/actions
+
+.. todo:: are unit tests currently working
+
+..
+   How to add a unit test
+   ----------------------
+
+   Unit test brings some advantages, briefly, we can list some of them (reference
+   [1](https://en.wikipedia.org/wiki/Unit_testing)):
+
+   * Tests reduce bugs in new features;
+   * Tests reduce bugs in existing features;
+   * Tests are good documentation;
+   * Tests reduce the cost of change;
+   * Tests allow refactoring;
+
+   With those advantages in mind, it was required to choose a framework which fits
+   well with C/C++ code.  Hence, it was chosen (google test)
+   [https://github.com/google/googletest], because it is largely used and it is
+   relatively easy learn.
+
+   Install and configure google test (manually)
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+   In this section, you will see a brief description of how to install google
+   test. However, it is strongly recommended that you take a look to the official
+   repository (https://github.com/google/googletest) and refers to that if you
+   have any problem to install it. Follow the steps below:
+
+   * Install: cmake and pthread
+   * Clone google test project from: https://github.com/google/googletest and
+   enter in the project directory
+   * Inside project directory, type:
+
+   ```
+   cmake -DBUILD_SHARED_LIBS=ON .
+   make
+   ```
+
+   * After compilation, copy all "*.so" inside directory "googlemock" and
+   "googlemock/gtest" to "/usr/lib/"
+   * Done! Now you can compile your tests.
+
+   If you have any problem, go to the official repository to find help.
+
+   Ps.: Some distros already have googletest packed. If your distro supports it,
+   you can use it instead of compile.
+
+   Create a new unit test
+   ~~~~~~~~~~~~~~~~~~~~~~
+
+   If you want to add new unit tests for Yosys, just follow the steps below:
+
+   * Go to directory "yosys/test/unit/"
+   * In this directory you can find something similar Yosys's directory structure.
+   To create your unit test file you have to follow this pattern:
+   fileNameToImplementUnitTest + Test.cc. E.g.: if you want to implement the
+   unit test for kernel/celledges.cc, you will need to create a file like this:
+   tests/unit/kernel/celledgesTest.cc;
+   * Implement your unit test
+
+   Run unit tests
+   ~~~~~~~~~~~~~~
+
+   To compile and run all unit tests, just go to yosys root directory and type:
+   ```
+   make unit-test
+   ```
+
+   If you want to remove all unit test files, type:
+   ```
+   make clean-unit-test
+   ```