docs: create STYLEGUIDE.md (#164)

STYLEGUIDE.md

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+# Code Style Guide for the YANG Catalog Project
+
+For matters which aren't mentioned in this file, refer to [PEP8](https://peps.python.org/pep-0008/).
+
+## Line length
+
+All lines should be kept under 120 characters in length.
+
+## Strings
+Use single quotes for strings. If a string contains a single quote, using double quotes is acceptable.
+
+Use f-strings instead of `str.format()`, `%`-formatting, and string concatenation where applicable. For composing file paths, use `os.path.join()` instead of f-strings.
+
+## Naming
+
+Use `snake_case` for file, variable, attribute, function, and method names. Use `ALL_CAPS` for the names of constants. For class names, use `CamelCase`.
+
+## Line breaks and indentation
+
+Use 4 spaces for indenting standard code blocks.
+
+### Function signatures and calls
+
+When the arguments for a function signature or a function call do not fit on a single line, pass each argument on it's own line:
+
+```python
+def function(
+    first_argument: str,
+    second_argument: str,
+    third_argument: str,
+    fourth_argument: str,
+    fifth_argument: str
+) -> str
+
+function(
+    very_long_argument_name,
+    another_very_long_name,
+    other_function().method().attribute ,
+    fourth_argument, fifth_argument
+)
+```
+
+### Initializing lists, dicts and sets
+
+Where possible, initialize on a single line. When the elements don't fit on a single line, write each on it's own line. Prefer assigning in a single statement to creating an empty dict and individually assigning to fields.
+
+```python
+my_dict = {'key1': value1, 'key2': value2}
+my_list = [
+    very_long_element_name1,
+    very_long_element_name2,
+    very_long_element_name3,
+    very_long_element_name4,
+    very_long_element_name5
+]
+```
+
+## Documentation strings
+
+Use `"""` to delimit docstrings. For single line docstrings, keep the quotes on the same line.
+
+```python
+"""A single line docstring"""
+```
+
+For multi line docstrings, begin the text on a new line.
+
+```python
+"""
+This is an example.
+
+Multi-line documentation string
+"""
+```
+
+In function docstrings, when describing parameters use this syntax:
+```python
+"""
+This is a function.
+
+Arguments:
+    :param p1   (str) A string parameter.
+    :param p2   (int) An integer parameter.
+    :return     (bool) A bool return value
+"""
+```