Add pre-commit support for sanity checks

Use pre-commit framework [1] to run black and flake8 before the commit.
black and flake8 are managed by the pre-commit framework and they can be
run manually by the user using `pre-commit run` command.

Fix the code base with the help of black and flake8.
Fix import statements according to PEP8 guidelines [1]
Both tools have the following settings (specified in the pre-commit
configuration file):
* line length: 120 characters
* directory to exclude: ethosu/vela/tflite/ and ethosu/vela/ethos_u55_regs

Updated README.md on how to install pre-commit and how to run sanity checks.
Pipenv files have been updated including new dependencies for pre-commit.

[1]: https://www.python.org/dev/peps/pep-0008/#imports
[2]: https://github.com/pre-commit/pre-commit

Change-Id: I304d9fffdf019d390ffa396a529c8a7c2437f63d
Signed-off-by: Diego Russo <diego.russo@arm.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/README.md b/README.md
index 03ad7fec..4d7db1a5 100644
--- a/README.md
+++ b/README.md
@@ -110,3 +110,45 @@ Contributions are accepted under [Apache License 2.0](LICENSE.txt). Only submit
 
 ## License
 Vela is licensed under [Apache License 2.0](LICENSE.txt)
+## Contributions and Pull Requests
+
+Contributions are accepted under Apache-2.0. Only submit contributions where you have authored all of the code.
+
+### Sanity checks
+
+The Python codebase is PEP8 compliant with the exception of 120 characters line length.
+We run black and flake8 against the code base excluding "ethosu/vela/tflite/" and "ethosu/vela/ethos\_u55\_regs" directories because they are auto-generated by third party tools.
+Those tools are run using [pre-commit framework](https://pre-commit.com/). The configuration file is .pre-commit-config.yaml
+
+#### Install tools
+
+To install pre-commit, run the following:
+
+```
+pipenv install -e . --dev
+```
+
+After the installation, pre-commit is available in the virtual environment.
+
+#### Install the pre-commit hook
+
+To ease the development, we can run those sanity checks before committing the code.
+To install the git hook, run:
+
+```
+$ pre-commit install
+pre-commit installed at .git/hooks/pre-commit
+```
+
+The checks will be run before the commit: if one of them fails, you need to fix the code to make the checks pass.
+
+#### Run the sanity checks
+
+Those checks can be run manually. This can be achievied running the following
+```
+$ pre-commit run flake8 --all-files
+...
+$ pre-commit run black --all-files
+```
+
+If you don't specify anything after run, it will execute all the checks.