Add prominent mention of greenback, README / index.rst

Fixes: #4
Change-Id: Ief6f9bbf5c87947ae645988165ad5dc86d223dbb

Author: zzzeek

README.rst

@@ -23,6 +23,12 @@ directly::
 
     print(asyncio.run(awaitlet.async_def(asyncio_sleep)))
 
+awaitlet is spun out from SQLAlchemy's own `asyncio mediation layer
+<https://docs.sqlalchemy.org/en/latest/orm/extensions/asyncio.html>`_, with no
+dependencies on SQLAlchemy itself.  awaitlet may be compared with another
+equivalent library `greenback <https://pypi.org/project/greenback/>`_ which was
+released at roughly the same time as SQLAlchemy's asyncio API.
+
 awaitlet is intentionally fully compatible with SQLAlchemy's asyncio mediation
 layer, and includes API patterns for:
 

docs/build/index.rst

@@ -17,7 +17,11 @@ Awaitlet is a direct extract of SQLAlchemy's own `asyncio mediation layer
 dependencies on SQLAlchemy (but is also fully cross-compatible with
 SQLAlchemy's mediation layer).   This code has been in widespread production
 use in thousands of environments for several years, starting in 2020 with
-SQLAlchemy 1.4's first release.
+SQLAlchemy 1.4's first release.     The library provides for the identical use
+case as another library that was first released around the same time as
+SQLAlchemy 1.4 called `greenback <https://pypi.org/project/greenback/>`_, but
+as part of the SQLAlchemy project is guaranteed to remain cross-compatible with
+non-blocking SQLAlchemy code.
 
 awaitlet without any dependency or use of SQLAlchemy includes API patterns for: