Removed obsolete dev guide.

[dblock]

I came here to try to repro ruby-grape/grape#2247 and bundle install failed with the following.

osx ~/source/rspec-expectations/rspec (main)$ bundle
Fetching https://github.com/rspec/rspec-core.git
Fetching https://github.com/rspec/rspec-mocks.git
Fetching https://github.com/rspec/rspec-support.git
Fetching gem metadata from https://rubygems.org/.........
Could not find gem 'rspec' in source at `/Users/dblock/source/rspec-expectations/rspec`.
The source does not contain any versions of 'rspec'

The instructions in DEV-README.md are incorrect and the file has typos (environment). Removed it in favor of the generated DEVELOPMENT.md.

Fixed README on how to contribute to rspec-expectations with export USE_GIT_REPOS=1, without rspec-dev.

[pirj]

The old way still works for me:

$ git clone https://github.com/rspec/rspec-expectations.git
$ cd rspec-expectations
$ bundle
Fetching https://github.com/rspec/rspec-support.git
Fetching https://github.com/rspec/rspec-mocks.git
Fetching https://github.com/rspec/rspec-core.git
Fetching https://github.com/rspec/rspec.git
Fetching gem metadata from https://rubygems.org/.........
...

USE_GIT_REPOS is not exported.

I suspect this check is failing for you.

You have the repo checked out in ~/source/rspec-expectations/rspec, and that confuses this condition. Check it out to ~/source/rspec-expectations, and it will work fine.

[dblock]

Thanks. I PRed typos fixes in #1360.