Replace p_context_init() with p_context_new()/p_context_delete()

UPGRADING.md

@@ -1,6 +1,17 @@
+## v4.0.0
+
+### API Changes
+
+- Replace any calls to `p_context_init()` with `p_context_new()`.
+- Replace any references to the address of a statically allocated context
+  structure with the pointer returned from `p_context_init()` (e.g. `&context`
+  -> `context`).
+- Add a call to `p_context_delete()` (for C or C++) after lexing/parsing to
+  reclaim context memory.
+
 ## v3.0.0
 
-### Grammar changes
+### Grammar Changes
 
 - Rename `ast;` statement to `tree;`.
 - Rename `ast_prefix;` statement to `tree_prefix;`.