Add readme file

Author: leocomelli

README.md

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+# Vault Backup
+
+:warning: Check [the oficial way](https://learn.hashicorp.com/tutorials/vault/sop-backup) to backup your HashiCorp Vault.
+
+Create a backup file of all HashiCorp Vault kv2 secrets.
+
+```bash
+./vault-backup -help
+  -base64
+        encode secret value as base64
+  -filename string
+        output filename (default "vault.backup")
+  -help
+        show this help output
+  -output string
+        output format. one of: json|yaml|kv (default "json")
+  -paths string
+        comma-separated base path. must end with /
+```
+
+Some environment variables must be defined before execution:
+
+* `VAULT_TOKEN`: is required, retrieve one by running `vault login`;
+* `VAULT_ADDR`: default value is `http://127.0.0.1:8200`
+
+## Example
+
+```bash
+./vault-backup -base64 -filename my-vault.backup
+
+2021/11/03 23:13:42 - reading production/app1/database
+2021/11/03 23:13:42 - reading production/app1/cache
+2021/11/03 23:13:43 - reading production/app2/database
+2021/11/03 23:13:44 done! ;)
+```
+
+Backup file
+
+```json
+{
+  "secret/data/production/app1/database/user": "dmF1bHQtYmFja3VwCg==",
+  "secret/data/production/app1/database/password": "czNjcjN0Cg==",
+  "secret/data/production/app1/cache/token": "czNjcjN0X19Ub2tlbgo=",
+  "secret/data/production/app2/database/user": "dmF1bHQtYmFja3VwCg==",
+  "secret/data/production/app2/database/password": "czNjcjN0Cg=="
+}
+```
+