Add coding guidelines document

CONTRIBUTING.md

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+Solana Coding Guidelines
+===
+
+The goal of these guidelines is to improve developer productivity by allowing developers to
+jump any file in the codebase and not need to adapt to inconsistencies in how the code is
+written. The codebase should appear as if it had been authored by a single developer. If you
+don't agree with a convention, submit a PR patching this document and let's discuss! Once
+the PR is accepted, *all* code should be updated as soon as possible to reflect the new
+conventions.
+
+Rust coding conventions
+---
+
+* All Rust code is formatted using the latest version of `rustfmt`. Once installed, it will be
+  updated automatically when you update the compiler with `rustup`.
+
+* All Rust code is linted with Clippy. If you'd prefer to ignore its advice, do so explicitly:
+
+  ```rust
+  #[cfg_attr(feature = "cargo-clippy", allow(too_many_arguments))]
+  ```
+
+  Note: Clippy defaults can be overridden in the top-level file `.clippy.toml`.
+
+* For variable names, when in doubt, spell it out. The mapping from type names to variable names
+  is to lowercase the type name, putting an underscore before each capital letter. Variable names
+  should *not* be abbreviated unless being used as closure arguments and the brevity improves
+  readability. When a function has multiple instances of the same type, qualify each with a
+  prefix and underscore (i.e. alice_keypair) or a numeric suffix (i.e. tx0).
+
+* For function and method names, use `<verb>_<subject>`. For unit tests, that verb should
+  always be `test` and for benchmarks the verb should always be `bench`. Avoid namespacing
+  function names with some arbitrary word. Avoid abreviating words in function names.
+
+* As they say, "When in Rome, do as the Romans do." A good patch should acknowledge the coding
+  conventions of the code that surrounds it, even in the case where that code has not yet been
+  updated to meet the conventions described here.
+
+
+Terminology
+---
+
+Inventing new terms is allowed, but should only be done when the term is widely used and
+understood. Avoid introducing new 3-letter terms, which can be confused with 3-letter acronyms.
+
+Some terms we currently use regularly in the codebase:
+
+* hash: n. A SHA-256 Hash
+* keypair: n. A Ed25519 key-pair, containing a public and private key.
+* pubkey: n. The public key of a Ed25519 key-pair.
+* sigverify: v. To verify a Ed25519 digital signature.
+