adjust readme

Author: RogerKSI

README.md

@@ -8,5 +8,8 @@ The standard libraries for building BandChain oracle scripts. Code in this packa
     - `oei` - a library containing functions for querying data from BandChain
 
 ## Coverage test
-- `owasm-crypto` - cargo tarpaulin --out Html --output-dir reports/crypto --packages owasm-crypto
-- `owasm-vm` - cargo tarpaulin --out Html --output-dir reports/vm --packages owasm-vm
+- Install tarpaulin by this command
+    - `cargo install cargo-tarpaulin`
+- Get coverage test results
+    - `owasm-crypto` - cargo tarpaulin --out Html --output-dir reports/crypto --packages owasm-crypto
+    - `owasm-vm` - cargo tarpaulin --out Html --output-dir reports/vm --packages owasm-vm