Document workflow based on the discussion in fortran-lang#5

Author: certik

WORKFLOW.md

@@ -1,5 +1,37 @@
 # Workflow for the Fortran stdlib contributors
 
-This document will describe the workflow we'll follow when developing the Fortran stdlib.
-It's largely to be discussed and decided.
-For now, take a look at the [issues](https://github.com/fortran-lang/stdlib).
+This document describes our current workflow. You are welcome to propose
+changes in it by opening an
+[issue](https://github.com/fortran-lang/stdlib/issues).
+
+1. You have an idea or a proposal. Open an issue to discuss it. This is on the
+   level of "is there interest in having image reader/writer functions in
+   stdlib?"
+
+2. When there seems to be significant interest in the proposal, like 80/20
+   participants think it's a good/bad idea, move on to discuss the specific
+   API. It's OK to propose the API off the bat if you already have an idea for
+   it.
+
+3. Discuss the API and iterate. When there is ~80/20 approval for the API, move
+   on to implement it and submit a PR. Small PRs are always better than large.
+   It's OK to implement only a few functions of a new module, and continue work
+   on the others in a later PR. All new functionality goes into an
+   "experimental" namespace (`stdlib_experimental_*.f90`).
+
+4. When opening a PR, request reviews from one or more people that are most
+   relevant to it. These are likely to be people involved in prior steps of the
+   workflow. Other contributors (not explicitly invited) are encouraged to
+   provide reviews and suggestions as well. Iterate until all (or most)
+   participants are on the same page. We should not merge if there is a strong
+   objection from the reviewers or from somebody in the wider community.
+
+5. Moving from experimental to main. Once new functionality lands into
+   "experimental", the next step is to write a "specification". The
+   specification is a document that describes the API and the functionality, so
+   that anyone can use it to create an implementation from scratch without
+   looking at `stdlib`. `stdlib` provides the reference implementation. When
+   this specification document is approved by the wide community and the
+   standards committee (informally), then we can move the code into main, and
+   the particular specification document becomes part of the Fortran Standard
+   Library.