chore(docs): add install schema doc

docs/install-spec.md

@@ -0,0 +1,51 @@
+# Install Plan File Schema
+
+Install plans define how to install agents, integrations, and instrumentation for a Quickstart. They’re defined once in the /install directory and then are referencable in Quickstart’s config.yml by using the install-plan field.
+
+## Filename format
+
+Install Plans are placed under `install/<plan_type>/install.yml` and can be nested.
+
+For eample:
+
+* `install/apm/java/install.yml`
+* `install/infrastructure/infra-agent-standard/install.yml`
+
+## Schema Definition
+
+```yaml
+id: string, required
+
+# where the install occurs
+# set up context/expectation for what the user is going to need to perform this installation
+target: object, required
+  type: enum, required                # one of [ agent, integration, on-host-integration, pixie, NPM (net perf monitoring), unknown (special case for guided-install) ]
+  destination: enum, required         # one of [ application, host, kubernetes, cloud, unknown (special-case for guided-install) ]
+  os: list(enum), optional            # one of [ linux, darwin, windows ]
+  context: string(markdown), optional # For display in the right-sidebar
+
+# primary install directive
+install: object, required
+  mode: enum, required            # one of: [ guidedInstall, targetedInstall, nerdlet, link ] (eventually: documentation, other fields (in-product help XP?) )
+  destination: object, required   # required input for the mode to function
+    recipeName: string, optional  # valid for mode: [ guidedInstall, targetedInstall ]
+    nerdletId: id, optional       # only valid for mode: nerdlet, ex: nerdpackId.nerdletId
+    url: string, optional         # valid for mode: link
+
+# secondary install directive
+# Note: even though this object is the same shape as `install`, we're choosing
+# to make it an explicit field, vs alternatives like having a single `install`
+# field that's a list(object) type (as that would be implicit behavior, and
+# I'd rather avoid that)
+fallback: object, optional
+  mode: enum, required            # one of: [ guidedInstall, targetedInstall, nerdlet, link ] (eventually: documentation, other fields (in-product help XP?) )
+  destination: object, required   # required input for the mode to function
+    recipeName: string, optional  # valid for mode: [ guidedInstall, targetedInstall ]
+    nerdletId: id, optional       # only valid for mode: nerdlet, ex: nerdpackId.nerdletId
+    url: string, optional         # valid for mode: link
+
+# Add more data fields
+# These fields are displayed in the install plan 
+title: string, optional       # Display-only field for install plan
+description: string, optional # Display-only field for install plan
+```