Add more design documents (#742)

* Add more design documents

This change adds a set of documents about the design/implementation of the Slang compiler intended for helping to onboard new contributors, explain some of the choices that go into the current implementation, and outline the broad strokes of some of our future feature goals.

As explained in the `README` for these docs, I don't expect them to be kept in sync with the code in the long run; there is no expectation that people making a PR for code changes also go and fix up these documents.
The purpose of these is to add a bit of a "historical record" of design choices and project goals to the project itself, so they can be browsed in a way that is more convenient than the issue tracker or source control history.

The "Coding Conventions" document is very much a first attempt, and I'm not 100% wedded to any of the decisions in there.
We haven't enforced consistent conventions so far, but the importance of having something in place will only grow with the number of contributors.

* Edits based on review

* Fix many typos (thanks @jsmall-nvidia)

* Change up some language in coding conventions based on discussions since when it was originally written. I'm still not 100% sure about some of the choices in there, but we need to get *something* established.

1 files changed, 21 insertions, 0 deletions

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+Slang Design and Implementation Notes
+=====================================
+
+This directory contains documents that are primarily intended for developers working on the Slang implementation.
+They are not indended to be helpful to Slang users.
+
+These documents can only be trusted to reflect the state of the codebase or the plans of their authors at the time they were written. Changes to the implementation are not expected to always come with matching changes to these documents, so some amount of drift is to be expected.
+
+Developers interested in contributing to Slang might want to start with the [Overview](overview.md) document, which describes the overall compilation pipeline that Slang uses and the purpose of the various steps (both implemented and planned).
+
+The [Coding Conventions](coding-conventions.md) document describes the conventions that should be followed in all code added to the Slang project.
+
+The [Interfaces](interfaces.md) document describes the high-level design plan for Slang's interfaces and generics features.
+
+The [Declaration References](decl-refs.md) document is intended to help out developers who are mystified by the heavily used `DeclRef` type in the compiler implementation.
+
+The [Intermediate Representation (IR)](ir.md) document describes the design of Slang's internal IR.
+
+The [Existential Types](existential-types.md) document goes into some detail about what "existential types" are in the context of the Slang language, and explains how we may go about supporting them.
+
+The [Capabilities](capabilities.md) document explains the proposed model for how Slang will support general notions of profile- or capability-based overloading/dispatch.