feature269 Describe intended functionality of env vars in MANUAL.

SVLINT_INCDIRS, SVLINT_PREFILES, SVLINT_POSTFILES

md/manual-introduction.md

@@ -119,6 +119,39 @@ All loaded plugins, via the `--plugin` option, are enabled and have access
 to all values in the TOML configuration.
 
 
+## Environment Variables
+
+Svlint is sensitive to 4 environment variables:
+- `SVLINT_CONFIG` - an exact absolute or relative path to a TOML configuration.
+  This is the only way to specify an *exact* configuration path, as the `-c`
+  command-line option is used to search hierarchically.
+- `SVLINT_INCDIRS` - a colon-separated list of include paths, i.e. an
+  alternative to using the `-I` command-line option.
+- `SVLINT_PREFILES` - a colon-separated list of files to process before those
+  given on the command-vline.
+- `SVLINT_POSTFILES` - a colon-separated list of files to process after those
+  given on the command-line.
+
+
+For example:
+```sh
+svlint -I/cad/tools/uvm -I/work/myTeam \
+  uvm_macros.svh \
+  /another/thing/first \
+  foo.sv bar.sv \
+  /another/thing/last \
+  check_pp_final_state.svh
+```
+
+Is exactly equivalent to:
+```sh
+$ export SVLINT_INCDIRS="/cad/tools/uvm:/work/myTeam"
+$ export SVLINT_PREFILES="uvm_macros.svh:/another/thing/first"
+$ export SVLINT_POSTFILES="/another/thing/last:check_pp_final_state.svh"
+$ svlint foo.sv bar.sv
+```
+
+
 ## Configuration
 
 Firstly, you need a TOML configuration file to specify which rules to enable.