Update README.md (#760)
Update README.md (#760)

* Update README.md

* Adds a disclaimer about Cargo environment variables not available as documented in Cargo

https://github.com/rust-lang/cargo/issues/8842
https://github.com/alexliesenfeld/httpmock/issues/45

* Update README.md

* Fixes a typo

* Update README.md

* Clarifies expected behaviour under environment variables: Dep fix PR 764 for Doctests

* Update README.md

* Expected behaviour fix

* Update README.md

* Update README.md

* Update README.md

* Update README.md

Review incorporated

* Update README.md

* Update README.md
xd009642/tarpaulin,xd009642/tarpaulin