Add a crashkernel-howto.txt doc
    
    Resolves: bz1974638
    Upstream: Fedora
    Conflict: None
    
    commit 7dbbb4bb31b6311e1283d0f801a26e7b6409248e
    Author: Kairui Song <kasong@redhat.com>
    Date:   Fri Jun 25 04:01:45 2021 +0800
    
        Add a crashkernel-howto.txt doc
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Baoquan He <bhe@redhat.com>
    
    Signed-off-by: Kairui Song <kasong@redhat.com>