clean up README a bit

README.md

@@ -1,16 +1,26 @@
 # ifupdown-ng
 
-This package is a work in progress implementation of the ifupdown suite.  It is
+ifupdown-ng is a network device manager that is largely compatible with Debian
-intended to be largely compatible with ifupdown and ifupdown2, with some caveats:
+ifupdown, BusyBox ifupdown and Cumulus Networks' ifupdown2.
+
+## Dependency Resolution
+
+![Dependency resolution example](doc/img/dependency-resolution.png)
+
+ifupdown-ng uses a dependency resolver to determine interface bring-up order
+in a deterministic way.  To use this feature, you must declare relationships
+between interfaces with the `requires` keyword.  This keeps the implementation
+efficient.  For convenience, the `requires` keyword is exported to executors
+as the `IF_REQUIRES` environment variable.
+
+For compatibility with some legacy ifupdown executors, we also provide the
+`requires` keyword under other environment variables in some cases.
+
+## Caveats
 
 * ifupdown2 python plugins are not supported at this time.  An executor could be
   written to handle them.
 
-* ifupdown-ng uses a SAT solver to determine interface bring-up order, like
-  ifupdown2.  However, relationships must be explicitly defined instead of
-  inferred by plugins in ifupdown2.  This simplifies the executors and ensures
-  consistent behaviour across executors.
-
 * ifupdown-ng retains compatibility with /etc/network/if-X.d scripts, but will
   prefer using executors in /usr/libexec/ifupdown-ng where appropriate.