Preparing changes for build for 1.10 (1st of many)

[sdavtaker]

Issue #, if available: 1888

Description of changes:

• Preparing instructions for installation in README.md to reflect changes to the cmake build scripts.
• Setting required cmake version to be 3.12 and dropping the warning about using 3.12.
• This changes are intended to be merged to build-dev-1.10 as a WIP to read a sane cmake experience. More PRs coming in the next days. I will link description of the project as Issue soon.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
  No test, only line of code changed is for required cmake version.
• Checked if this PR is a breaking (APIs have been changed) change.
  No an API breaking changing, but forces upgrade of CMAKE for those having older than 3.12 in their toolchain. We were warning in every build about it since a year ago, and we are going to hold this from reaching main branch until version 1.10 is released.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

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