Document the state.dat file a little bit

README.md

@@ -136,6 +136,8 @@ In this directory you will find
 - `datadir` Once your sbws scanner has started gathering results, it will dump
   them into this directory. Other sbws commands (such as generate and stats)
   read results from the files in this directory.
+- `state.dat` A file for storing state needed between sbws commands. See its
+  documentation for more information.
 
 ## Running tests
 

docs/source/index.rst

@@ -25,6 +25,7 @@ Contents
    faq
    glossary
    diagrams
+   state
    sbws
 
 Proposals

docs/source/sbws.util.rst

@@ -28,6 +28,14 @@ sbws.util.parser module
     :undoc-members:
     :show-inheritance:
 
+sbws.util.state module
+~~~~~~~~~~~~~~~~~~~~~
+
+.. automodule:: sbws.util.state
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
 sbws.util.stem module
 ~~~~~~~~~~~~~~~~~~~~~
 

docs/source/state.rst

+The ``state.dat`` file
+======================
+
+This file contains state that multiple sbws commands may want access to and
+that needs to persist across processes. Both read and write access to this file
+is wrapped in the ``State`` class, allowing for safe concurrent access: the
+file is locked before reading or writing, and (for now) only simple data types
+are allowed so we can be sure to update the state file on disk every time the
+state is modified in memory.
+
+At the time of writing, the following fields can exist in the state file.
+
+``scanner_started``
+-------------------
+
+The last time ``sbws scanner`` was started.
+
+- **Producer**: ``sbws scanner``, once at startup.
+
+- **Consumer**: ``sbws generate``, once each time it is ran.