Updates style guidelines

CONTRIBUTING.md

@@ -3,10 +3,10 @@
 The following is a set of guidelines for contributing to PISA.
 
 ## Code Style Guidelines
-We use [black](https://github.com/psf/black) as our base code formatter. Before submitting a PR make sure you have properly formatted your code by running:
+We use [black](https://github.com/psf/black) as our base code formatter with a line length of 12 chars. Before submitting a PR make sure you have properly formatted your code by running:
 
-```
-black {source_file_or_directory}
+```bash
+black --line-length=120 {source_file_or_directory}
 ```
 On top of that, there are a few rules to also have in mind.
 
@@ -30,6 +30,23 @@ else:
     missed_confirmations[tx] = 1
 ```
 
+An exception to the rule are nested `if` statements that placed right after each other and `if` statements with a single line of code.
+
+```python
+for opt, arg in opts:
+    if opt in ["-s", "server"]:
+        if arg:
+            pisa_api_server = arg
+```
+
+```python
+if rcode == 0:
+    rcode, message = self.check_start_time(start_time, block_height)
+if rcode == 0:
+    rcode, message = self.check_end_time(end_time, start_time, block_height)
+```
+
+
 ## Code Documentation
 Code should be, at least, documented using docstrings. We use the [Sphinx Google Style](https://www.sphinx-doc.org/en/master/usage/extensions/example_google.html#example-google) for documenting functions.