[course][exercise] added info on params.input as new alternative default input file name

exercises/exercise-runtimeparams/README.md

@@ -26,12 +26,13 @@ Parameters::init(argc, argv);
 ```
 
 This input file should either be named the same as the executable file, with a
-trailing `*.input`, as is standard in our CMake system, or be explicitly written
+trailing `*.input`, or be named `prarams.input` as is standard in our CMake system.
+Alternatively, arbitrarily named input file names can be explicitly written
 as the first shell argument after the executable file is called.
 
 ```bash
 ./exercise_runtimeparams
-    (Calls a file (exercise_runtimeparams.input) as the default input file.)
+    (Calls a file (exercise_runtimeparams.input, or, if this is not present, params.input) as the default input file.)
 ```
 ```bash
 ./exercise_runtimeparams exercise1.input