docs: update README.md with information about signy

Updates the README.md with information about the signy library.

Signed-off-by: Daniel Mangum <[email protected]>

Author: hasheddan

README.md

@@ -1,3 +1,61 @@
 # signy
 
 Signed URLs for small devices.
+
+## Background
+
+`signy` is a library for generating signed URLs on embedded devices using
+asymmetric cryptography. Signed URLs are typically utilized to grant
+time-limited access to private resources, such as those served by a [Content
+Delivery Network (CDN)](https://en.wikipedia.org/wiki/Content_delivery_network).
+In the context of embedded devices, signed URLs may be used to enable an
+external system, or another component in the same system, to access a specific
+resource. For example, a device may generate a signed URL and pass it to a more
+capable system to allow it to download the resource on its behalf.
+
+Use of `signy` requires a server that is capable of verifying the signed URLs it
+generates. If using `signy` with [Golioth](https://golioth.io), the certificates
+associated with the Certificate Authority (CA) that issued the device
+certificate must be [uploaded to your
+project](https://docs.golioth.io/connectivity/credentials/pki#integrating-with-golioth).
+
+## Usage
+
+The `signy` repository is a [Zephyr
+module](https://docs.zephyrproject.org/latest/develop/modules.html) and can be
+included in any Zephyr project by adding the following to the project's
+`west.yml` file.
+
+```yaml
+- name: signy
+  path: modules/lib/signy
+  revision: main
+  url: https://github.com/golioth/signy.git
+```
+
+`signy` leverages the [Platform Security Architecture (PSA) Crypto
+API](https://arm-software.github.io/psa-api/crypto/) for signing operations.
+Private keys must be
+[generated](https://arm-software.github.io/psa-api/crypto/1.3/api/keys/management.html#c.psa_generate_key)
+or
+[imported](https://arm-software.github.io/psa-api/crypto/1.3/api/keys/management.html#c.psa_import_key)
+using PSA Crypto APIs, and a signed certificate containing the public key must
+be passed to the `signy` library.
+
+Signed URLs generated by `signy` adhere to the following format.
+
+```
+BASEURL?nb=NOTBEFORE&na=NOTAFTER&cert=CERTIFICATE&sig=SIGNATURE
+```
+
+- `BASEURL`: the location of the resource (e.g.
+  `https://gw.golioth.io/.u/c/[email protected]`)
+- `NOTBEFORE` / `NOTAFTER`: the unix timestamps defining the window in which the
+  signed URL is valid. `signy` uses the device system time for `NOTBEFORE` and
+  sets the `NOTAFTER` timestamp based on `CONFIG_SIGNY_URL_VALIDITY_DURATION`.
+- `CERTIFICATE`: the base64 unpadded URL encoding of the device certificate.
+- `SIGNATURE`: the signature over the entire portion of the URL preceding
+  `&sig=`.
+
+For more information on how to use `signy`, see the [examples](./examples)
+directory.