Clarify that the migrate command populates the cache with data

It wasn't obvious from the documentation that the REST-API
/plugins/cache-chroniclemap/migrate
was generating the new configuration and also copying the associated
data from H2.

Add a paragraph making the statement explicit and highlighting
in details what the command does.

Change-Id: I1073cd040d1512a4ba3f935afcc2e60a2d5f8a9c

src/main/resources/Documentation/migration.md

diff --git a/src/main/resources/Documentation/migration.md b/src/main/resources/Documentation/migration.md
index 22ebb2c..0370232 100644
--- a/src/main/resources/Documentation/migration.md
+++ b/src/main/resources/Documentation/migration.md
@@ -12,6 +12,12 @@
 database, it will participate to the overall database load, so you should test
 accordingly.
 
+The migration would do the following:
+1. scan all existing cache key-value pairs
+2. calculate the parameters for the new cache
+3. create the new cache
+4. read all existing key-value pairs and insert them into the new cache-chroniclemap files
+
 The following caches will be migrated (if they exist and contain any data):
 
 * accounts