docs: document the vesting event schema

[bobthecomputer]

Closes #2

Summary

• document schema version 1.0.0 for all four current contract events
• list every topic and positional data field with its Soroban type and meaning
• add decoded JSON examples, numeric precision guidance, and indexer ordering rules
• define a schema versioning policy and link the document from the README

Verification

• cross-checked all four event symbols and layouts against contracts/vesting/src/lib.rs
• cargo test --locked: 4 passed, 0 failed
• git diff --check: passed

The examples are explicitly decoded views. They do not imply that Stellar RPC returns named JSON fields instead of XDR values.