Add deprecation policy to the zcashd book.

doc/book/src/SUMMARY.md

@@ -6,6 +6,7 @@
 - [Developer Documentation](dev.md)
   - [Rust in `zcashd`](dev/rust.md)
   - [Regtest tips and hints](dev/regtest.md)
+  - [Deprecation Policy](dev/deprecation.md)
 - [Design](design.md)
   - [Chain state](design/chain-state.md)
   - ["Coins" view](design/coins-view.md)

doc/book/src/dev/deprecation.md

@@ -0,0 +1,28 @@
+Deprecation Policy
+==================
+
+From time to time, features of zcashd and its associated wallet and RPC API are deprecated
+to allow eventual removal of functionality that has been superseded by more recent
+improvements. Deprecation follows a process whereby deprecate features can be explicitly
+turned on or off using the `-allowdeprecated=<feature>` CLI argument.
+
+Zcashd internally supports two sets of deprecated feature flags in `src/deprecation.h`:
+- `DEFAULT_ALLOW_DEPRECATED` contains the set of features that remain available for
+  use without having to be specifically enabled using `-allowdeprecated`.
+- `DEFAULT_DENY_DEPRECATED` contains the set of features that are not enabled by
+  default, and must be explicitly enabled using `-allowdeprecated`.
+
+Deprecation occurs as a 3-step process:
+- When a feature is initially deprecated in a release, the fact of its deprecation is
+  announced in the release notes, any functionality that supersedes the deprecated feature
+  (if any) is documented in the release notes, and the string `DEPRECATED` is added to
+  user-facing API documentation and CLI help text. Internally, the deprecated feature
+  added to `DEFAULT_ALLOW_DEPRECATED`. The feature remains available to end-users
+  without additional action on their part.
+- After 3 minor-version releases, or 1 major-version release, the deprecated feature
+  is removed from `DEFAULT_ALLOW_DEPRECATED` and added to `DEFAULT_DENY_DEPRECATED`.
+  After this point, the user must explicitly enable the feature using an
+  `-allowdeprecated` zcash.conf or CLI parameter in order for the feature to remain
+  available to them.
+- After at least 2 major-version releases, the deprecated feature may be removed entirely. 
+