doc(README): update README for the latest commits

Author: Damian Rouson

README.md

@@ -9,40 +9,45 @@ Overview
 ========
 
 DAG is a Fortran 2018 library for creating and manipulating directed acyclic graphs (DAGs).  
-DAG is based on a fork of [daglib by Jacob Williams], refactored to add
+DAG is based on a fork of [daglib by Jacob Williams], refactored to 
 
-* A build system and test harness automated by [fpm],
-* Unit testing written with [Vegetables], 
-* Continuous-integration testing and documentation deployment via [GitHub Actions], 
-* Documentation generated by [FORD], 
-* Runtime assertion-checking using [Assert], and
-* [JSON] file input/output using [jsonff].  
+* Adopt a functional programming pattern based on (mostly) pure function constructors
+  and component getters but no setters.  When compilers cooperate, we recommend using 
+  `associate`  to assign names to constructor results to avoid unnecessary copies.
+* Add build system and test harness automated by [fpm],
+* Add unit testing written with [Vegetables], 
+* Add continuous-integration testing and documentation deployment via [GitHub Actions], 
+* Add ocumentation generated by [FORD], 
+* Add runtime assertion-checking using [Assert], and
+* Add [JSON] file input/output using [jsonff].  
 
-DAG includes a topological sort feature, and it generates files in the [GraphViz] "dot" format.
+Constructor results contain an private index array that describes a topologically sorting
+of the vertices in a DAG.
 
 Prerequisites
 -------------
-DAG was developed with the following prerequisite package versions:
+The [fpm] package manager automatically downloads and builds all dependencies.
+See [fpm.toml] for a complete list, including version numbers.
 
-1. [gfortran] 10.2.0
-2. [OpenCoarrays]  2.9.2
-3. [fpm] 0.1.3
-4. [graphviz] 2.44.1
-
-Earlier versions might work also.
-
-Building and testing
+Downloading, building and testing
 --------------------
-After installing [fpm], clone, build, and test, execute the following in a `bash`-like shell:
+With [fpm]  installed, clone
 ```
 git clone [email protected]:sourceryinstitute/dag
-fpm test --compiler caf --runner "cafrun -n 1"
 ```
-replacing `1` in the last line with the desired number of images to execute in parallel for
-each test.  Also try compiling and running the example with
+### Serial build and testing
+Build and test the `dag` library  with in a `bash`-like shell as follows:
+```
+fpm test
 ```
-fpm run --example
+### Parallel building and testing with the gfortran/OpenCoarrays
+Build and test the `dag` library  with in a `bash`-like shell as follows:
 ```
+fpm test --compiler caf --runner "cafrun -n 2"
+```
+replacing `2` in the last line with the desired number of images to execute in parallel
+for each test.  
+
 Please report any test failures by submitting an [issue] on the DAG repository.
 
 Examples