Add project structure documentation

docs/Project.md

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+# Project Files
+
+Cryptol supports specifying a *project* file that can accelerate the repeated
+loading and testing of a large, interconnected set of source files. Cryptol
+will remember the hashes of the files previously checked and use this to avoid
+type-checking and testing files that are unchanged since the previous loading
+of the project.
+
+To use this feature a `cryproject.toml` should be created in the root directory
+of the cryptol source files.
+
+To check a whole project Cryptol can be invoked with the `--project` or `-p`
+flag using the directory containing the project as an argument.
+
+Example:
+
+```shell
+cryptol -p myproject/
+```
+
+To discard the previous cached results and reload a whole project use
+`--refresh-project`. This can be useful when versions of external tools
+have changed or simply to get confidence that everything is still in a
+working state.
+
+Example:
+
+```shell
+cryptol -p myproject/ --refresh-project
+```
+
+## `cryproject.toml` Format
+
+Project files are described by a [TOML](https://toml.io/en/) file using the
+following top-level key-value assignments:
+
+- `root` can optionally be specified to override the directory that Cryptol
+  files are located in.
+
+- `modules` is a list of filenames containing the leaf modules in a project.
+  These modules, and all of their dependencies, will be loaded when the project
+  is loaded.
+
+Example file:
+
+```toml
+modules = [
+    "Example/A.cry",
+    "Example/B.cry",
+]
+```
+
+## `loadcache.toml` Format
+
+After loading a project a cache file is generated and stored in
+`.cryproject/loadcache.toml`.  This file contains a version number to allow
+caches to automatically invalidate when the project subsystem updates.
+
+- `version` specifies the cache file format version in order to allow old
+  caches to be invalidated when Cryptol changes the meaning of this file.
+
+- `file` specifies the absolute path to a Cryptol module for those stored
+  in files.
+
+- `memory` specifies the module name of a primitive module built into Cryptol.
+
+- `fingerprint` specifies a SHA2-256 hash of the source file which is used to
+  detect when the source file has changed from the previous run.
+
+- `foreign_fingerprints` is a list of SHA2-256 hashes of shared libraries that
+  this Cryptol file directly includes.
+
+- `include_fingerprints` is a list of SHA2-256 hashes of pre-processor included
+  files that this Cryptol files directly includes.
+
+- `docstring_result` is a boolean `true` when `:check-docstrings` previously
+  succeeded for this module and `false` when it previously failed. It will be
+  missing if tests were never run on this module.
+
+```toml
+version = 1
+
+[[modules]]
+fingerprint = "2f671b21f2933a006b6a843c7f281388e6b8227f9944b111f87711dc9ca8448f"
+foreign_fingerprints = []
+include_fingerprints = []
+memory = "Cryptol"
+
+[[modules]]
+docstring_result = true
+file = "/path/to/project/Id.cry"
+fingerprint = "a9e6f7a4b65ead6bd8e27442717d6b0dc54afc73e34b18c32f005ceb7a8f3c34"
+foreign_fingerprints = [ "c7767a13281a56631c72b9b6f69a17746dc02213e7f2b24a8a4a6fe7afd9ee0a" ]
+include_fingerprints = []
+
+[[modules]]
+docstring_result = true
+file = "/path/to/project/Main.cry"
+fingerprint = "6b36f965ebb1a68cf76d689a966806ec879540aa6576a76c1aaa7705a4af09d5"
+foreign_fingerprints = []
+include_fingerprints = []
+```