docs: add ADR for mergable preflight specs (#858)

Adds a proposed Architecture Decision Records for the ability to specify
multiple specs per preflight test.

Author: xavpaice

docs/arch/adr-002-mergeable-preflights.md

@@ -0,0 +1,37 @@
+# ADR 002: Mergeable preflight specs
+
+When the `preflight` binary, or pacakge, is called, it supports only one spec definition at a time.
+
+Recent changes in Troubleshoot allow the `support-bundle` binary to be called with multiple specs at a time.  This allows cluster components to contribute independant Troubleshoot specs for their scope, and have Troubleshoot assemble them at run time.
+
+Tools such as kURL have components which are maintained with a degree of separation to one another.  It would be helpful to the maintainers of such projects to be able to call `preflight` specifying a number of specs at runtime, allowing Troubleshoot to assemble them into one spec for collection/analysis.
+
+Currently if the `preflight` binary is called with multiple specs, it simply ignores all after the first.
+
+## Decision
+
+Modify the `preflight` CLI and package to be able to read multiple args rather than just one.
+
+Introduce a merge mechanism in the same way that the `support-bundle` binary runs, to merge and deduplicate Preflight specs.
+
+## Status
+
+Proposed
+
+## Consequences
+
+There are no backward compatibility consequences or breaking changes in this proposal.
+
+The project benefits:
+* Folks maintaining kURL add-ons can contribute unique preflight specs for their add-on (and the same for other projects simlarly structured)
+* Folks using `preflight` from the CLI without other applications (e.g. for a Helm install) are able to specify a list of preflights for their application rather than having to assemble one spec for each environment.
+
+## Design notes
+
+This proposal does not include adding the `uri:` field to `kind: Preflight`.
+
+The file `cmd/preflight/cli/root.go` calls `preflight.RunPreflights` with `args[0]` which is likely to need to change to just `args`.
+
+Func `RunPreflights` takes a single string arg (`arg string`) for the spec definition.  This is likely to need to change to `arg []string`.
+
+File `cmd/troubleshoot/cli/run.go` loops through the list of args, concatenating them together.  A similar process is suitable for this change.