doc: Update README.md to include INCUBATING status

Based on the EP for the stable module ABI I think
an additional status would be useful to call out
those which we believe are ready for final review
versus those that we expect to iterate on before
that final review should take place.

Author: mhdawson

README.md

@@ -58,15 +58,25 @@ All EP documents must be MIT licensed.
 ## Progress of an EP
 
 All EPs will be committed to the repository regardless of their acceptance.
-The initial status shall be **"DRAFT"**.
+The initial status shall either be **"INCUBATING"** or **"DRAFT"**.
+
+If the status is **"INCUBATING"** that means that it is expected that the
+EP will be iterated on over a period of time before it becomes a
+complete proposal such that could be reviewed and move to **"ACCEPTED"**.
+This status allows visiblity and feedback while setting the appropriate
+expectation in terms of completeness.
+
+If the status is **"DRAFT"**, that means the authors believe the prosoal
+is complete and is ready to be reviewed in the context of deciding to
+move it to **"ACCEPTED"** or **"REJECTED"**.
 
 If the document is uncontroversial and agreement is reached quickly it might be
 committed directly with the **"ACCEPTED"** status. Likewise, if the proposal is
 rejected the status shall be **"REJECTED"**. When a document is rejected a
 member of the core team should append a section describing the reasons for
 rejection.
 
-A document shall also be committed in **"DRAFT"** status. This means consensus
-has not been reached yet.
+A document shall also be committed in **"INCUBATING"** or **"DRAFT"**
+status. This means consensus has not been reached yet.
 
 The author of an EP is expected to actually pursue and implement the proposal.