Contributions by @instagibbs to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad

_includes/ref_wallets.md

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+## Wallets 
+
+### Deterministic Wallet Formats
+
+#### Type 1: Single Chain Wallets
+
+{% autocrossref %}
+
+Type 1 deterministic wallets are the simpler of the two, which can
+create a single series of keys from a single seed. A primary weakness is
+that if the seed is leaked, all funds are compromised, and wallet
+sharing is extremely limited.
+
+{% endautocrossref %}
+
+#### Type 2: Hierarchical Deterministic (HD) Wallets
+
+{% autocrossref %}
+
+![Overview Of Hierarchical Deterministic Key Derivation](/img/dev/en-hd-overview.svg)
+
+For an overview of HD wallets, please see the [developer guide
+section][devguide wallets].  For details, please see BIP32.
+
+{% endautocrossref %}