docs(readme): add boost version requirements

[mojomex]

PR Type

• Improvement

Description

When trying to install Nebula on Ubuntu 20.04 with manually built Humble, a lot of Boost compile errors were emitted for
big_float32_buf_t and load_little_u16 etc. which are only supported for Boost 1.74.0 and later. Add this requirement to the readme file to avoid trouble in the future.

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[drwnz]

LGTM