Contributing

Please keep the scope of this project in mind. Dutyl is about tool integration - if your contribution is not using D tools, and can be run on a machine that doesn't have any of the D tools installed, it's probably not a good fit for Dutyl, and you should try sending it to the d.vim plugin instead. For example, syntax highlighting based on Vim regular syntax highlighting facilities doesn't belong here, but syntax highlighting based on some tool that uses libdparse or something it might be a good fit.
At the beginning of doc/dutyl.txt there is a line that specifies the current version of Dutyl. When there are changes in the code but a new version is not yet released, there should be a + after the version - e.g. Version: 1.2.3+ - to indicate that this is a different, mid-version. If that + is not there already, please add it as part of the PR.

Architecture outline

Dutyl is designed as a plugin framework, and it's user facing functions - like autocompletion or jump-to-definition - are implemented as plugins bundled in the release.
A plugin usually focus on a single tool - like Dub or DCD - and exposes functions that utilize that tool - like importPaths or complete. The exposed functions should be tool-agnostic - for example both Dscanner and DCD expose declarationsOfSymbol which accepts the same arguments and returns result in the same format.
Designed as a plugin framework
Each plugin focus on a tool(like Dub or DCD) and expose the tool's functionality as Vim functions
User-facing functions(accessible via commands) request functions from the plugins, and use them to fulfil the users' requests