docs(plus): create "checkpoint" version of subject

subjects/devops/plus-checkpoint/README.md

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+## plus
+
+### Instructions
+
+In this exercise, you will make a script `plus.sh` that will take two arguments from the command line, add them and output the result.
+
+### Usage
+
+```console
+$ ./plus.sh 2 3
+5
+$
+```