bump version for development (#3281)

* bump version for development

* Update .appveyor.yml

* Update README.rst

.appveyor.yml

-version: 3.0.0rc1.{build}
+version: 3.0.0.{build}
 
 image: Visual Studio 2015
 platform: x64

VERSION.txt

-3.0.0rc1
+3.0.0

python-package/README.rst

@@ -34,6 +34,12 @@ Install `wheel <https://pythonwheels.com>`_ via ``pip install wheel`` first. Aft
 
     pip install lightgbm
 
+The 3.0 version is pre-released, you can install it via:
+
+.. code:: sh
+
+   pip install lightgbm==3.0.0rc1
+
 Build from Sources
 ******************