Add docs for secp256k1 and instruction introspection. (#12369)

docs/src/implemented-proposals/instruction_introspection.md

@@ -0,0 +1,28 @@
+---
+title: instruction introspection
+---
+
+## Problem
+
+Some smart contract programs may want to verify that another Instruction is present in a
+given Message since that Instruction could be be performing a verification of certain data,
+in a precompiled function. (See secp256k1\_instruction for an example).
+
+## Solution
+
+Add a new sysvar Sysvar1nstructions1111111111111111111111111 that a program can reference
+and received the Message's instruction data inside, and also the index of the current instruction.
+
+Two helper functions to extract this data can be used:
+
+```
+fn load_current_index(instruction_data: &[u8]) -> u16;
+fn load_instruction_at(instruction_index: usize, instruction_data: &[u8]) -> Result<Instruction>;
+```
+
+The runtime will recognize this special instruction, serialize the Message instruction data
+for it and also write the current instruction index and then the bpf program can extract the
+necessary information from there.
+
+Note: custom serialization of instructions is used because bincode is about 10x slower
+in native code and exceeds current BPF instruction limits.