Add readme info for calling pre & post checks (#106)

While writing and testing my first migration, it took me a while to
realize the hooks for
UncheckedOnRuntimeUpgrade::pre_upgrade/post_upgrade weren't running due
to the --disable-mbm-checks flag not being set.

This PR adds a bit of extra information to the readme that would have
been helpful.

See code - [flag
logic](https://github.com/paritytech/try-runtime-cli/blob/b45be7dfce89fd881be27171d3ea114ef19d832c/core/src/commands/on_runtime_upgrade/mod.rs#L135)
Introduced in #90 
@liamaharon

---------

Co-authored-by: Alexandre R. Baldé <alexandrer_b@outlook.com>

Author: PolkadotDom

README.md

@@ -135,6 +135,10 @@ after the migration. [`OnRuntimeUpgrade::pre_upgrade`] returns a [`Vec<u8>`] tha
 arbitrary encoded data (usually some pre-upgrade state) which will be passed to
 [`OnRuntimeUpgrade::pre_upgrade`] after upgrading and used for post checking.
 
+**Note on Multi-Block Migrations (MBM):** If the runtime uses MBMs, the standard 
+`pre_upgrade` and `post_upgrade` checks might be skipped by the executive. To 
+force these hooks to run synchronously for testing, use the `--disable-mbm-checks` flag.
+
 ### [`VersionedMigration`]
 
 It is strongly suggested to use [`VersionedMigration`] when writing custom migrations for
@@ -182,6 +186,8 @@ cargo build --features try-runtime --release && cp target/release/substrate .
 try-runtime \
     --runtime /path-to-substrate/target/release/wbuild/my-runtime.wasm \
     on-runtime-upgrade \
+    # Passing this flag will skip multi-block-migration checks and only run pre_upgrade/post_upgrade checks.
+    --disable-mbm-checks \
     live --uri ws://localhost:9999
 ```