Merge pull request #634 from MVrachev/rewrite-readme

docs: rewrite README - continuation

Author: jku

README.md

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+# securesystemslib
+
+[![CI](https://github.com/secure-systems-lab/securesystemslib/workflows/Run%20Securesystemslib%20tests/badge.svg)](https://github.com/secure-systems-lab/securesystemslib/actions?query=workflow%3A%22Run+Securesystemslib+tests%22+branch%3Amain)
+
+Securesystemslib is a cryptography interface for signing and verifying digital
+signatures. It is developed for the [TUF](https://theupdateframework.io) and
+[in-toto](https://in-toto.io) projects: the key and signature containers are
+compatible with metadata formats from those projects.
+
+Under the hood, Securesystemslib can use various digital signing systems
+(e.g. [cryptography](https://pypi.org/project/cryptography/), PIV hardware keys
+and multiple cloud-based key management systems).
+
+## Installation
+
+The default installation supports [pure-Python `ed25519` signature
+verification](https://github.com/pyca/ed25519) only. To enable other schemes and
+signature creation, `securesystemslib` can be installed with *extras*. See
+[pyproject.toml](pyproject.toml) for available *optional dependencies*.
+
+```bash
+# Install with ed25519, RSA, ECDSA sign and verify support
+pip install securesystemslib[crypto]
+```
+
+```bash
+# ...or with HSM (e.g. Yubikey) support
+pip install securesystemslib[hsm]
+```
+
+## Usage
+[python-securesystemslib.readthedocs.io](https://python-securesystemslib.readthedocs.io)
+
+## Contact
+- Questions and discussions:
+  [`#securesystemslib-python`](https://cloud-native.slack.com/archives/C05PF3GA7AL)
+  on [CNCF Slack](https://communityinviter.com/apps/cloud-native/cncf)
+- Security issues: [*Report a vulnerability*](https://github.com/secure-systems-lab/securesystemslib/security/advisories/new)
+- Other issues and requests: [*Open a new
+  issue*](https://github.com/secure-systems-lab/securesystemslib/issues/new)
+
+## Testing
+`tox` is used for testing. It can be installed via
+[pip](https://tox.wiki/en/4.9.0/installation.html#via-pip) and executed from the
+command line in the root of the repository.
+
+```bash
+tox
+```