docs(concat_string): add subject description

subjects/devops/concat-string/README.md

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+## concat_string
+
+### Instructions
+
+Create a file `concat_string.py` that contains a function `concat` which takes in two strings as parameters and performs the following operations:
+
+- Concatenates the two strings by adding a comma and space between them.
+- Returns the resulting string.
+
+### Usage
+
+Here is an example of how to use the `concat` function:
+
+```python
+import concat_string
+string1 = "Hello"
+string2 = "World"
+print(concat_string.concat(string1, string2))
+```
+
+This will output:
+
+```console
+'Hello, World'
+```
+
+### References
+
+[string concatenation](https://www.w3schools.com/python/ref_string_concatenation.asp)