Update the instructions for building from source in README.md

[adolfogc]

Issue #, if available:

The current instructions for building from source can be improved.

Description of changes:

Updates the instructions for building from source, noting that an additional step is required so that the package is installed with the correct version number.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[xiazhvera]

Thanks for the contribution!
We intentionally set the version number as 1.0.0-dev to distinguish a local build from the released versions. It's not required to set the version number while building from source.
I will make the following update and bring in the changes:

• Marking the version part as optional
• Create a dedicate section to explain how to set versions

[adolfogc]

Hi, thank you for the reply. The version string is not optional in the case of the awscrt package when you have to build it from source as a dependency for the iot-sdk package. Otherwise it gets rebuilt, which could be problematic if you had built it using the system's libcrypto, as it wouldn't be the case for the second time it gets build.

[xiazhvera]

I added more explanation to the version strings and build instruction. Hopefully, it would be clearer about the issue with the version string. Feel free to comment with feedback.
https://github.com/aws/aws-iot-device-sdk-python-v2/pull/477/files

[xiazhvera]

Closed as the change is merged in at: #477