Documentation missing information about target folder

[paolobettelini]

The custom build script for leptonic assumes that:

• the target folder is named "target";
• the target folder is located in the project root.

This may not be the case. Thus, IMO the installation section of the documentation should mention that you cannot set CARGO_TARGET_DIR to anything other than the default.
For the first problem, I don't think we have an actually reliable way to get the target folder in the build script. The second problem may be fixable.