doc: expand discussion of compiler status

Author: rouson

README.md

@@ -66,12 +66,14 @@ to finalize objects.  The table below summarizes the observed compiler behaviors
 
 | _Compiler_   | _Test failures_      | _Version tested_                                    |
 | :---         |       :---:          | :---                                                |
-| NAG          |         1            | `nagfor` 7.1                                        |
-| GCC          |         4            | `gfortran` 11.3.0, 12.1.0                           |
+| NAG          |         0            | `nagfor` 7.1 Build 7113                             |
+| GCC          |         6            | `gfortran` 12.2.0                                   |
 | Intel        |         2            | `ifort` 2021.5.0 Build 20211109\_000000             |
 | NVIDIA       | Fails to build (ICE) | `nvfortran` 2022.2                                  |
 | AMD          | Fails to build (ICE) | `flang` 13.0.0 (AOCC_3.2.0-Build\#128 2021\_11\_12) |
 
+See the [test suite README.md](./test/README.md) for more details on each compiler's test
+failures.
 
 Downloading, Building, and Testing
 ----------------------------------

test/README.md

@@ -0,0 +1,39 @@
+Compiler Support Status
+=======================
+
+This directory contains two categories of unit tests separated into two files:
+* `compiler_test.f90` checks whether a compiler performs finalizes objects in 
+  each scenario that the Fortran 2018 standard requires. 
+* `usage_test.f90` exercises the reference-counter library.
+The compiler tests verify the compiler in isolation, making _no_ use of the
+reference-counter library.
+
+`nagfor` 7.1.0
+--------------
+The Numerical Algorithms Group [(NAG]) Fortran compiler] passes all of the tests in the
+reference-counter test suite. :trophy:
+
+`gfortran` 12.2.0
+-----------------
+Because the first usage test below causes a segmentation fault,
+exposing `gfortran`'s test failures requires running each test individually
+as follows:
+```
+fpm test -- -f "<description>"
+```
+where one must replace <description> above with one of the enumerated
+descriptions below (or a corresponding unambiguous substring).
+
+Failing checks in `compiler_test.f90`:
+1. finalizes a non-allocatable object on the LHS of an intrinsic assignment
+2. finalizes an allocated allocatable LHS of an intrinsic assignment
+3. finalizes a function reference on the RHS of an intrinsic assignment
+4. finalizes a function reference on the RHS of an intrinsic assignment
+5. finalizes a specification expression function result
+
+Failing checks in `usage_test.f90`:
+1. copy points to the same resource as the original
+2. has zero references after a shallow copy goes out of scope
+
+[(NAG) Fortran compiler]: https://www.nag.com/content/nag-fortran-compiler
+