Add API-level documentation for the piped_processt class.

Author: NlightNFotis

doc/API/util/piped_process.md

@@ -0,0 +1,25 @@
+# `src/util/piped_process.{cpp, h}`
+
+To utilise the `piped_process` API for interprocess communication with any binary:
+
+* You need to initialise it by calling the construct `piped_processt("binary with args", message_handler)`.
+  * A `message_handlert` should usually be present down from the call stack at 
+    the place you want to call `piped_process` from.
+  * If IPC fails during child process creation, you will get a `system_exceptiont`.
+  * If the `binary command` does not correspond to a binary in the `$PATH` or is
+    not a path to a binary itself, you'll read a string `Launching <xyz> failed with error: <error>`
+    when you attempt to `receive()` output from it.
+* To close the underlying child process, you need a call to the destructor - `~piped_processt()`.
+  * This will occur automatically when the `piped_processt` goes out of scope if
+    it's locally scoped.
+* Use `send()` to send a string message to the child process' input.
+  * This returns a `send_responset`, an enum that shows whether the
+    sending of the message through the pipe succeeded or failed.
+* Use `receive()` to read a string message from the child process' output.
+  * It's always a good idea to guard a call to `receive` with `can_receive()`,
+    so that receiving is blocked until there's something to receive.
+    * `can_receive` with no arguments will default to infinite wait time for piped
+      process readiness.
+  * Alternatively, you can guard the call to `receive` with `wait_receivable`.
+   `wait_receivable` takes an integer value representing `microseconds` of waiting
+   time between checks for pipe readiness.