provide binaries of nimble, add info about locking to docs
I went into [some problems](https://forum.nim-lang.org/t/9222) when trying to built my program on different machine. I was advised to use nimble's lock functionality (which solved my problem).

However:

- there is no mention (in main documentation) that lock functionality is available only in version 0.14.0 (I needed to lock into changelog)
- I needed to manually build new version of nimble on both of my machines (you should provide binaries at least for windows)