Add style guide

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+# Gitaly code style
+
+## Errors
+
+### Use %v when wrapping errors
+
+Use `%v` when wrapping errors with context.
+
+    return fmt.Errorf("foo context: %v", err)
+
+### Use %q when interpolating strings
+
+Unless it would lead to incorrect results, always use `%q` when
+interpolating strings. The `%q` operator quotes strings and escapes
+spaces and non-printable characters. This can save a lot of debugging
+time.
+
+## Tests
+
+### Table-driven tests
+
+We like table-driven tests ([Cheney blog post], [Golang wiki]).
+
+-   It should be clear from error messages which test case in the table
+    is failing. Consider giving test cases a `name` attribute and
+    including that name in every error message inside the loop.
+-   Use `t.Errorf` inside a table test loop, not `t.Fatalf`: this
+    provides more feedback when fixing failing tests.
+
+  [Cheney blog post]: https://dave.cheney.net/2013/06/09/writing-table-driven-tests-in-go
+  [Golang wiki]: https://github.com/golang/go/wiki/TableDrivenTests