lint: add markdown linter (#5254)

abci/README.md

@@ -27,11 +27,10 @@ A detailed description of the ABCI methods and message types is contained in:
 
 To compile the protobuf file, run (from the root of the repo):
 
-```
+```sh
 make protoc_abci
 ```
 
 See `protoc --help` and [the Protocol Buffers site](https://developers.google.com/protocol-buffers)
 for details on compiling for other languages. Note we also include a [GRPC](https://www.grpc.io/docs)
 service definition.
-

abci/example/kvstore/README.md

@@ -21,11 +21,10 @@ The state is persisted in leveldb along with the last block committed,
 and the Handshake allows any necessary blocks to be replayed.
 Validator set changes are effected using the following transaction format:
 
-```
+```md
 "val:pubkey1!power1,pubkey2!power2,pubkey3!power3"
 ```
 
 where `pubkeyN` is a base64-encoded 32-byte ed25519 key and `powerN` is a new voting power for the validator with `pubkeyN` (possibly a new one).
 To remove a validator from the validator set, set power to `0`.
 There is no sybil protection against new validators joining. 
-