add doc/mod/ipcalc

(cherry picked from commit e9953c3612)

doc/mod/ipcalc.md

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+IP address calculation
+======================
+
+[◀ Go back to main README](../../README.md)
+
+> ℹ️️ **Info**: This module can not be used on its own but requires the base
+> installation. See [main README](../../README.md) for details.
+
+Description
+-----------
+
+This module adds functions for IP address calculation.
+
+Requirements and installation
+-----------------------------
+
+Just install the module:
+
+    $ScriptInstallUpdate mod/ipcalc;
+
+Usage and invocation
+--------------------
+
+### IPCalc
+
+The function `$IPCalc` prints information to terminal, including:
+
+* address
+* netmask
+* network in CIDR notation
+* minimum host address
+* maximum host address
+* broadcast address
+
+It expects an IP address in CIDR notation as argument.
+
+    $IPCalc 192.168.88.1/24;
+
+![IPCalc](ipcalc.d/ipcalc.avif)
+
+### IPCalcReturn
+
+The function `$IPCalcReturn` expects an IP address in CIDR notation as
+argument as well. But it does not print to terminal, instead it returns
+the information in a named array.
+
+    :put ([ $IPCalcReturn  192.168.88.1/24 ]->"broadcast");
+
+![IPCalcReturn](ipcalc.d/ipcalcreturn.avif)
+
+---
+[◀ Go back to main README](../../README.md)  
+[▲ Go back to top](#top)