Merge pull request #2930 from nim-lang/contributing-guide

contribution guide

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+The git stuff
+=============
+
+`Guide by github, scroll down a bit <https://guides.github.com/activities/contributing-to-open-source/>`_
+
+Deprecation
+===========
+
+Backward compatibility is important, so if you are renaming a proc or
+a type, you can use
+
+
+.. code-block:: nim
+
+  {.deprecated [oldName: new_name].}
+
+Or you can simply use
+
+.. code-block:: nim
+
+  proc oldProc() {.deprecated.}
+
+to mark a symbol as deprecated. Works for procs/types/vars/consts,
+etc.
+
+`Deprecated pragma in the manual. <http://nim-lang.org/docs/manual.html#pragmas-deprecated-pragma>`_
+
+Writing tests
+=============
+
+Not all the tests follow this scheme, feel free to change the ones
+that don't. Always leave the code cleaner than you found it.
+
+Stdlib
+------
+
+If you change the stdlib (anything under ``lib/``), put a test in the
+file you changed. Add the tests under an ``when isMainModule:``
+condition so they only get executed when the tester is building the
+file. Each test should be in a separate ``block:`` statement, such that
+each has its own scope. Use boolean conditions and ``doAssert`` for the
+testing by itself, don't rely on echo statements or similar.
+
+Sample test:
+
+.. code-block:: nim
+
+  when isMainModule:
+    block: # newSeqWith tests
+      var seq2D = newSeqWith(4, newSeq[bool](2))
+      seq2D[0][0] = true
+      seq2D[1][0] = true
+      seq2D[0][1] = true
+      doAssert seq2D == @[@[true, true], @[true, false], @[false, false], @[false, false]]
+
+Compiler
+--------
+
+The tests for the compiler work differently, they are all located in
+``tests/``. Each test has its own file, which is different from the
+stdlib tests. At the beginning of every test is the expected side of
+the test. Possible keys are:
+
+- output: The expected output, most likely via ``echo``
+- exitcode: Exit code of the test (via ``exit(number)``)
+- errormsg: The expected error message
+- file: The file the errormsg
+- line: The line the errormsg was produced at
+
+An example for a test:
+
+.. code-block:: nim
+
+  discard """
+    errormsg: "type mismatch: got (PTest)"
+  """
+
+  type
+    PTest = ref object
+
+  proc test(x: PTest, y: int) = nil
+
+  var buf: PTest
+  buf.test()
+
+Running tests
+=============
+
+You can run the tests with
+
+.. code-block:: bash
+
+  ./koch tests
+
+which will run a good subset of tests. Some tests may fail. If you
+only want to run failing tests, go for
+
+.. code-block:: bash
+
+  ./koch tests --failing all
+
+You can also run only a single category of tests. For a list of
+categories, see ``tests/testament/categories.nim``, at the bottom.
+
+.. code-block:: bash
+
+  ./koch tests c lib
+
+Comparing tests
+===============
+
+Because some tests fail in the current ``devel`` branch, not every fail
+after your change is necessarily caused by your changes.
+
+The tester can compare two test runs. First, you need to create the
+reference test. You'll also need to the commit id, because that's what
+the tester needs to know in order to compare the two.
+
+.. code-block:: bash
+
+  git checkout devel
+  DEVEL_COMMIT=$(git rev-parse HEAD)
+  ./koch tests
+
+Then switch over to your changes and run the tester again.
+
+.. code-block:: bash
+
+  git checkout your-changes
+  ./koch tests
+
+Then you can ask the tester to create a ``testresults.html`` which will
+tell you if any new tests passed/failed.
+
+.. code-block:: bash
+
+  ./koch --print html $DEVEL_COMMIT