Bring contributing content from tw.org into dev docs

docs/contrib/coding_style.md

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+---
+title: Coding Style
+---
+
+The coding style used for the Taskwarrior, Taskserver, and other codebases is
+deliberately kept simple and a little vague. This is because there are many
+languages involved (C++, C, Python, sh, bash, HTML, troff and more), and
+specіfying those would be a major effort that detracts from the main focus which
+is improving the software.
+
+Instead, the general guideline is simply this:
+
+Make all changes and additions such that they blend in perfectly with the
+surrounding code, so it looks like only one person worked on the source, and
+that person is rigidly consistent.
+
+To be a little more explicit, the common elements across the languages are:
+
+-   Indent code using two spaces, no tabs
+-   With Python, follow [PEP8](https://www.python.org/dev/peps/pep-0008/) as
+    much as possible
+-   Surround operators and expression terms with a space
+-   No cuddled braces
+-   Class names are capitalized, variable names are not
+
+We target Python 2.7 so that our test suite runs on the broadest set of
+platforms. This will likely change in the future and 2.7 will be dropped.
+
+We can safely target C++11 because all the default compilers on our supported
+platforms are ready. Feel free to use C++14 and C++17 provided that all build
+platforms support this.