Add motivation to README (#47)

README.md

@@ -1,6 +1,39 @@
 # `puffin`
 
-An experimental Python package manager.
+An experimental Python packaging tool.
+
+## Motivation
+
+Puffin is an extremely fast (experimental) Python package resolver and installer, intended to
+replace `pip` and `pip-tools` (`pip-compile` and `pip-sync`).
+
+Puffin itself is not a complete "package manager", but rather a tool for locking dependencies
+(similar to `pip-compile`) and installing them (similar to `pip-sync`). Puffin can be used to
+generate a set of locked dependencies from a `requirements.txt` file, and then install those
+locked dependencies into a virtual environment.
+
+Puffin represents an intermediary goal in our pursuit of building a "Cargo for Python": a Python
+package manager that is extremely fast, reliable, and easy to use -- capable of replacing not only
+`pip`, but also `pipx`, `pip-tools`, `virtualenv`, `tox`, `setuptools`, and even `pyenv`, by way of
+managing the Python installation itself.
+
+Puffin's limited scope allows us to solve many of the low-level problems that are required to
+build such a package manager (like package installation) while shipping an immediately useful tool
+with a minimal barrier to adoption. Try it today in lieu of `pip` and `pip-tools`.
+
+## Limitations
+
+Puffin does not yet support:
+
+- Source distributions
+- VCS dependencies
+- URL dependencies
+- Uninstalling packages
+- ...
+
+Like `pip-compile`, Puffin generates a platform-specific `requirements.txt` file (unlike, e.g.,
+`poetry`, which generates a platform-agnostic `poetry.lock` file). As such, Puffin's
+`requirements.txt` files are not portable across platforms and Python versions.
 
 ## Usage
 
@@ -16,6 +49,23 @@ To install from a resolved `requirements.txt` file:
 cargo run -p puffin-cli -- sync requirements.txt
 ```
 
+For more, see `cargo run -p puffin-cli -- --help`:
+
+```text
+Usage: puffin-cli <COMMAND>
+
+Commands:
+  compile  Compile a `requirements.in` file to a `requirements.txt` file
+  sync     Sync dependencies from a `requirements.txt` file
+  clean    Clear the cache
+  freeze   Enumerate the installed packages in the current environment
+  help     Print this message or the help of the given subcommand(s)
+
+Options:
+  -h, --help     Print help
+  -V, --version  Print version
+```
+
 ## Benchmarks
 
 ### Resolution