doc: Add idea for composable functions

doc/draft-composable-functions.md

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+# Composable functions
+
+## Example
+
+### With pipes
+
+    musala-pull SOURCE \
+      | musala-add TASK \
+      | musala-remove --purge SOMETASK \
+      | musala-push DEST
+
+### With internal filters
+
+Also writeable like
+
+    musala pull SOURCE % add TASK1 % remove --purge TASK2 % push DEST
+
+
+## Commands
+
+### musala pull
+
+Download all tasks from source
+
+Get tasks from remote source
+
+Writes tasks as JSON to stdout
+
+### musala add
+
+Add new task
+
+Get tasks from stdin (json)
+
+Add a new task
+
+Writes tasks as JSON to stdout
+
+### musala remove
+
+* Get tasks from stdin (json)
+* Mark a task as removed
+* Remove tasks marked as remove (if --purge enabled)
+* Writes tasks as JSON to stdout
+
+### musala push
+
+* Get tasks from stdin (json)
+* Writes tasks to REMOTE DESTINATION
+
+