docs: add adr for lowercase infohashes

adrs/20230510152112_lowercas_infohashes.md

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+# Lowercase infohashes
+
+## Description
+
+We use both uppercase and lowercase infohashes. This is a problem because
+we have to check both cases. For example, we have to convert to uppercase before
+inserting into the database or querying the database.
+
+The database and API URLs use uppercase infohashes, and they are case-sensitive.
+
+## Agreement
+
+We agree on use lowercase infohashes everywhere and try to convert then as soon
+as possible from the input.
+
+There is no specific reason to use lowercase infohashes, but we have to choose
+one of them. We decided to use lowercase because the infohash is a hash, and
+hashes are usually lowercase.
+
+We will change them progressively.

adrs/README.md

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+# Architectural Decision Records (ADRs)
+
+This directory contains the architectural decision records (ADRs) for the
+project. ADRs are a way to document the architectural decisions made in the
+project.
+
+More info: <https://adr.github.io/>.
+
+## How to add a new record
+
+For the prefix:
+
+```s
+date -u +"%Y%m%d%H%M%S"
+```
+
+Then you can create a new markdown file with the following format:
+
+```s
+20230510152112_title.md
+```
+
+For the time being, we are not following any specific template.