update docs

Author: RCasatta

README.md

@@ -4,10 +4,12 @@
 
 Firma is a tool to create bitcoin multisig wallets with private keys stored on offline devices.
 
-The offline device could be a [CLI](cli) terminal or a spare [android](android) phone.
+The offline device could be a [CLI](cli) terminal or a spare [Android](android) phone.
 
 Information is transferred between devices through QR codes. Since PSBT could become large some kB, more than 1 QR code could be needed, those QRs are chained with QR [structured append](https://segno.readthedocs.io/en/stable/structured-append.html) 
 
+Wallets data are saved in the local filesystem (see [datadir](/docs/datadir.md)) and optionally encrypted with an externally provided key (on Android everything is encrypted by the default with a key protected by native keystore).
+
 It is based on:
   * [bitcoin core](https://bitcoincore.org/)
   * [PSBT](https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki) (Partially Signed Bitcoin Transaction)
@@ -119,7 +121,7 @@ Check the bin [readme](cli/README.md) for an example with CLI
   gpg --decrypt encryption_key.gpg | firma-offline --read-stdin random --key-name bitcoin-key
   ```
 
-  On android, system keystore is used to encrypt 32 random bytes, so that physical attacks need to break the secure element if the device has one.
+  On Android, system keystore is used to encrypt 32 random bytes, so that physical attacks need to break the secure element if the device has one.
 </details>
 
 <details>

docs/datadir.dot

@@ -0,0 +1,27 @@
+digraph G {
+  ".firma" -> "[network]"
+
+  "[network]" -> wallets
+  "[network]" -> keys
+  "[network]" -> psbts
+  "[network]" -> "daemon_opts"
+
+  keys -> "[key name]"
+  "master_secret" [shape=Square]
+  "descriptor_public_key" [shape=Square]
+  "[key name]" -> "master_secret"
+  "[key name]" -> "descriptor_public_key"
+
+  wallets -> "[wallet name]"
+  "wallet" [shape=Square]
+  "wallet_indexes" [shape=Square]
+  "daemon_opts" [shape=Square]
+  "wallet_signature" [shape=Square]
+  "[wallet name]" -> "wallet"
+  "[wallet name]" -> "wallet_indexes"
+  "[wallet name]" -> "wallet_signature"
+
+  psbts -> "[psbt name]"
+  "psbt" [shape=Square]
+  "[psbt name]" -> "psbt"
+}

docs/datadir.md

@@ -0,0 +1,9 @@
+# Datadir
+
+Firma operates saving data in a datadir which defaults to `$HOME/.firma` but could be configured.
+
+Datadir is organized like this.
+
+![datadir](datadir.svg)
+
+All squares are json files while circles are directories.

docs/datadir.svg

@@ -47,7 +47,6 @@ impl Display for Kind {
     }
 }
 
-
 impl FromStr for Kind {
     type Err = io::Error;