Add clarification to pre param

[aegershman]

As a pipeline plumber, I'd like to know how my pipelines can use the x.y.z-build.* semver scheme.

The current documentation is fuzzy (see [#68]) and makes it sound like alpha, beta, or rc are the only pre options. However, it turns out you can use any convention you'd like. Example:

jobs:
- name: build
  plan:
  - get: version
    params:
      pre: build # <---- x.y.z-build.*
...
  - get: version
    params:
      pre: my-amazing-string-prefix-convention # <--- x.y.z-my-amazing-string-prefix-convention.1

Although any string prefix can be used, maybe it shouldn't be advertised. Maybe only semi-conventional naming setups should be advertised, such as alpha, beta, rc, and build

See [#68].

[vito]

makes sense, thanks!