Clarify CONTRIBUTING.md and README.md instructions

[gmichelo]

Hi all,

I was following the instructions of CONTRIBUTING.md and README.md and I noticed few possible issues:

1. Correct me if I am wrong, but development tools such as the ones listed in submission-prerequisites section should be part of tools.go, right? Also, these could be all run automatically by a Makefile.

2. The below step from the instructions refers to a Makefile, right? I couldn't find such file.

Run make to build your changes and ensure the build is not broken.

3. The contributing's instructions do not tell to run the Trillion MySQL database. Though it is required for certain set of Go tests. It would be remind people to start their docker before running go test ./....

4. The readme says that Go 1.13 is required, but Go 1.14 seems to be actually required. For example, the T.Cleanup method is introduced only in Go 1.14: https://github.com/google/keytransparency/search?q=T.Cleanup&unscoped_q=T.Cleanup