README provides bad install advice

[jayhale]

This is a 2 year old edit, but it still seems worth cleaning up. The install instructions in the README fail so long as v3 is held in beta:

https://github.com/graphql-python/graphene-django/blob/5d5d7f18154adc0d89e2c6baebd47c4e5d7d9fbf/README.md?plain=1#L31

$ pip install "graphene-django>=3"
ERROR: Could not find a version that satisfies the requirement graphene-django>=3 (from versions: 1.0.dev20160909000001, 1.0.dev20160910000001, 1.0.dev20160917000001, 1.0.dev20160919000001, 1.0.dev20160919000002, 1.0.dev20160919000003, 1.0.dev20160919000004, 1.0.dev20160920000001, 1.0.dev20160922000001, 1.0, 1.1.0, 1.2.0, 1.2.1, 1.3, 2.0.dev2017072501, 2.0.dev2017072601, 2.0.dev2017073101, 2.0.dev2017083101, 2.0.0, 2.1rc0, 2.1rc1, 2.1.0, 2.2.0, 2.3.0, 2.3.2, 2.4.0, 2.5.0, 2.6.0, 2.7.0, 2.7.1, 2.8.0, 2.8.1, 2.8.2, 2.9.0, 2.9.1, 2.10.0, 2.10.1, 2.11.0, 2.11.1, 2.12.0, 2.12.1, 2.13.0, 2.14.0, 2.15.0, 3.0.0b1, 3.0.0b2, 3.0.0b3, 3.0.0b4, 3.0.0b5, 3.0.0b6, 3.0.0b7)
ERROR: No matching distribution found for graphene-django>=3