[ADD] issue template for proposals

[remiadon]

Issue template to follow every time
Example using this template to propose type indexing in BIDS-prov

Type-indexing on agents/activities/entities in BIDS-prov

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Problem Statement

When defining new graphs in BIDS-prov we have no way to know the type of an object in advance.
If we want to find all entities in a given graph, we must iterate over the entire graph.
In addition, this layout currently makes reading of the graph harder, and spotting errors becomes tedious

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Rationale

We propose to use a feature called type indexing, for all our graphs
This will allow to group nodes by type, and maybe to create a hierarchy in the future, accessing the most basic types at first and then descending in the graph for more specific types (eg. Agent vs Software Agent)

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Minimal examples

Before

"@graph": [
        {
          "@id" : "dfsiosndfn1",
          "label" : "realign",
          "@type" : "prov:Activity",
        },
]

After

"@graph": {
        "prov:Activity" : [
        {
          "@id" : "dfsiosndfn1",
          "label" : "realign",
        },
    ]
}

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Checklist

• link to the google doc of the proposal : here
• make sure the google doc follows the BIDS proposale template
• list of all authors for this proposal : Satra Ghosh, Camille Maumet
• links to related issues and PRs in bids-specification