Document use cases

[pablochacin]

xk6 has many use cases that are not evident from existing documentation.

Since #111 the build logic was implemented using k6foundry library.

This change has created multiple issues (#136 #139 #) because some use cases were not considered in the design of k6foundry

Use cases

• build k6 from a fork using XK6_K6_REPO environment variable (used in k6 CI for PR from external contributors)
• Use references to modules without the domain (e.g. use in synthetic monitoring go run ./cmd/xk6 build --with sm=../xk6-sm)
• Build private local modules (not obtained from a repository but from a local folder). Used in k6 CI for building test extensions from local directories