README: Remove out-of-date contribution guide

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7THI2

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The latest contribution guide is archived at
https://gitee.com/openeuler/kernel-docs and
https://gitee.com/openeuler/community/tree/master/sig/Kernel



Signed-off-by: Wei Li <liwei391@huawei.com>