[flang][docs] Document the `flang` wrapper script

[kiranchandramohan]

LGTM.

[Meinersbur]

LGTM

flang/docs/FlangDriver.md

149

flang/docs/FlangDriver.md

 invoking the compiler driver, `flang-new`, with the `-fc1` flag.
 
 The frontend driver will only run one action at a time. If you specify multiple
 action flags, only the last one will be taken into account. The default action
 is `ParseSyntaxOnlyAction`, which corresponds to `-fsyntax-only`. In other
 words, `flang-new -fc1 <input-file>` is equivalent to `flang-new -fc1 -fsyntax-only
 <input-file>`.
 
+## The `flang` script
+The `flang` wrapper script for `flang-new` was introduced as a development tool
+and to facilitate testing. While code-generation is not available in Flang, you
+can use it as a drop-in replacement for other Fortran compilers in your build
+scripts.
+
+The `flang` wrapper script will:
+* use `flang-new` to unparse the input source file (i.e. it will run `flang-new
+  -fc1 -fdebug-unparse <input-file>`), and then
+* call a host Fortran compiler, e.g. `gfortran`, to compile the unparsed file.
+
+Here's a basic breakdown of what happens inside `flang` when you run `flang
+file.f90`:
+```bash
+flang-new -fc1 -fdebug-unparse file.f90 -o file-unparsed.f90
+gfortran file-unparsed.f90
+```
+This is a simplified version for illustration purposes only. In practice,
+`flang` adds a few more frontend options and it also supports various other use
+cases (e.g. compiling C files, linking existing object files). `gfortran` is
+the default host compiler used by `flang`. You can change it by setting the
+`FLANG_FC` environment variable.
+
+Our intention is to replace `flang` with `flang-new`. Please consider `flang`
+as a temporary substitute for Flang's compiler driver while the actual driver
+is in development.
+
 ## Adding new Compiler Options
 Adding a new compiler option in Flang consists of two steps:
 * define the new option in a dedicated TableGen file,
 * parse and implement the option in the relevant drivers that support it.
 
 ### Option Definition
 All of Flang's compiler and frontend driver options are defined in
 `clang/include/clang/Driver/Options.td` in Clang. When adding a new option to