Create ADR documenting constants style guide agreements

docs/0001-declare-constants-at-the-top-of-the-file.md

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+# Constants are created by file as a top level declaration
+
+* Status: Accepted
+* Deciders: Marino, Zorica, Stefanija, Davide, Tomasz, Maciej
+* Date: 17/09/2021
+
+
+## Context and Problem Statement
+
+This is a minor code-style decision regarding where constants should be placed.
+
+
+## Considered Options
+
+1. Declare constants at the top of the file (right after imports)
+```
+import ...
+
+const val PUBLIC_CONSTANT="value"
+private const val CONSTANT="value"
+
+class { ... }
+```
+
+2. Declare constants at a file's level but at the bottom
+3. Declare constants in a companion object, at the bottom of the file
+
+## Decision Outcome
+Accepting the risks of name clashes that might turn out from making some private constants public when they are declared outside of companion objects (as evaluated low probability-low impact), we decided to go for Option #1.
+Such decison was based on the fact that we didn't see major differencies between the listed approaches and approach #1 is already the one with the wider usage in the codebase.
+