Add format documentation for the OTSMeta format

Signed-off-by: Knut Ahlers <knut@ahlers.me>

cmd/ots-cli/.gitignore

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+ots-cli

docs/OTSMeta-format.md

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+> OTS uses two different formats to store secrets under the hood. Both of them can be read and written by the frontend implementation as well as by the `ots-cli` application.
+
+## Simple Format
+
+The simple format is the format used by OTS since day one and is the, well, most simple one. It only consists of the secret encrypted using OpenSSL AES-256-CBC compatible encryption. This format is preferred for backwards compatibility when no other reasons require the use of the OTS-Meta format.
+
+```console
+# openssl enc -aes-256-cbc -pbkdf2 -md sha512 -iter 300000 -pass pass:12345678 -a -A <<<"I'm a secret"
+U2FsdGVkX19G3GuIw3LGM0PVQmavPU/LnWvJhcLeYvs=
+
+# curl -H 'Content-Type: application/json' -d '{"secret": "U2FsdGVkX19G3GuIw3LGM0PVQmavPU/LnWvJhcLeYvs="}' https://ots.fyi/api/create
+{"success":true,"expires_at":"2023-10-11T19:45:01.315587714Z","secret_id":"bbd53ec5-8ee9-4df5-a630-9561313a348a"}
+
+# ots-cli fetch "https://ots.fyi/#bbd53ec5-8ee9-4df5-a630-9561313a348a%7C12345678"
+INFO[0000] fetching secret...                           
+I'm a secret
+```
+
+## OTSMeta Format
+
+The OTSMeta format was first introduced in `v1.9.0` of OTS together with the possibility to attach files to the secret. It contains structured data with a banner to differentiate between a simple JSON shared through OTS and the OTSMeta format. The OTSMeta structure itself is a simple JSON document containing a secret and a number of attachments having their contents base64 encoded:
+
+```json
+{
+  "secret": "I'm a secret",
+  "attachments": [
+    {
+      "name": "file.txt",
+      "type": "text/plain",
+      "data": "SSdtIGZpbGUgY29udGVudAo="
+    }
+  ]
+}
+```
+
+This structure is prefixed with the Banner `OTSMeta` and then shared the same way as a simple secret would be:
+
+```console
+# ots-cli create -f file.txt <<<"I'm a secret"
+INFO[0000] reading secret content...                    
+INFO[0000] attaching file...                             file=file.txt
+INFO[0000] creating the secret...                       
+INFO[0000] secret created, see URL below                 expires-at="2023-10-11 19:52:30.816059504 +0000 UTC"
+https://ots.fyi/#6a6be08c-97d7-4970-a202-5bb6964460d8%7CwNUURZ0LRrQAhaczdZfj
+
+# curl -sS https://ots.fyi/api/get/6a6be08c-97d7-4970-a202-5bb6964460d8 | jq -r .secret >/tmp/secret.bin
+# openssl enc -aes-256-cbc -pbkdf2 -md sha512 -iter 300000 -pass pass:wNUURZ0LRrQAhaczdZfj -a -A -d </tmp/secret.bin
+OTSMeta{"secret":"I'm a secret\n","attachments":[{"name":"file.txt","type":"text/plain; charset=utf-8","data":"SSdtIGZpbGUgY29udGVudAo="}]}
+```
+
+Of course it's also possible to share a simple secret in OTSMeta format but the recommended way would be to omit the OTSMeta wrapping:
+```
+OTSMeta{"secret":"I'm a secret"}
+```
+
+When programmatically reading secrets you therefore need to check whether the secret starts with `OTSMeta` and decode the remaining as a JSON document and if it does not just use all the content as the secret.