Update release steps in README file

Add and update the release steps in README file:
 - Tag (and push) the new version;
 - Build source and wheel;
 - Sign the distribution files;
 - Upload with twine (more secure).

README.md

@@ -27,8 +27,28 @@ To report issues related to OWASP ZAP API, bugs and enhancements requests, use t
 
 ### Release to PyPi
 
-To build and upload to PyPi you can run the following:
+Example commands use the version `0.0.1`, it should be replaced accordingly to the version being released.
 
-    python setup.py sdist upload
+Tag (and push) the new version:
+
+    git tag -s 0.0.1 -m "Version 0.0.1."
+    git push upstream 0.0.1
+
+(Checkout the tag and ensure the working copy is clean.)
+
+Build the distribution (source and wheel) files:
+
+    python setup.py sdist bdist_wheel --universal
+
+Sign the distribution files:
+
+    gpg --detach-sign -a dist/python-owasp-zap-v2.4-0.0.1.tar.gz
+    gpg --detach-sign -a dist/python_owasp_zap_v2.4-0.0.1-py2.py3-none-any.whl
+
+Upload to PyPi:
+
+     twine upload \
+         dist/python-owasp-zap-v2.4-0.0.1.tar.gz python-owasp-zap-v2.4-0.0.1.tar.gz.asc \
+         dist/python_owasp_zap_v2.4-0.0.1-py2.py3-none-any.whl python_owasp_zap_v2.4-0.0.1-py2.py3-none-any.whl.asc
 
 The user must have permissions to upload to the `python-owasp-zap-v2.4` package.