Add a section about why this exists.

README.md

@@ -10,6 +10,54 @@ It was built with packages that are maintained with
 mind but it works in a more limited way with other
 approaches.
 
+What is the Problem?
+--------------------
+
+As developers, we would like to have a single source or truth for
+the version number of packages we maintain. In projects that use
+[poetry](https://github.com/python-poetry/poetry), 
+this is a `pyproject.toml` file, which typically begins 
+something like
+
+```
+[tool.poetry]
+name = "mypackage"
+version = "1.2.3"
+...
+```
+
+Once I have put a version number there, I would like the
+code
+
+```python
+import mypackage
+
+myppackage.version
+```
+
+to return the version number from `pyproject.toml`.
+
+The issue is that project versions go through different
+lifecycle phases. A package that has been built and published
+on [pypi](https://www.pypi.org) has the version number that
+started in `pyproject.toml` when it was built encoded elsewhere.
+But a newer version that I am actively developing, testing, or debugging
+before release only has the `pyproject.toml` version.
+
+`usingversion` knows about this, and knows how to find the version
+in either of those places in different scenarios. It makes 
+`mypackage.version` always work no matter how I am using or developing
+or testing.
+
+### What about the `+`
+
+As an added measure, to reduce confusion between development and 
+production environments, when the version number comes straight
+from `pyproject.toml` in a development environment, a `+` is 
+appended. This indicates that the source could have been modified
+in the development environment. So instead of `"1.2.3"`, the
+version will be reported as `"1.2.3+"`.
+
 For Package Maintainers
 -----------------------
 
@@ -48,6 +96,8 @@ For End Users
 -------------
 
 There's really nothing all that exciting to do.
+You might not even know or care that the developer
+of a package you depend on chose to use `usingversion`.
 If a package `somepackage` that you depend on uses
 `packageversion` then you can check the version with