Add documentation for the tx-tracker service (#642)

### Description

This pull request adds a README.md file explaining how the `tx-tracker` service works.

tx-tracker/README.md

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+# tx-tracker service
+
+The purpose of this service is to fetch, for each VAA, the metadata of associated to the transaction that generated it.
+
+## Data flow
+
+This service is consumes new VAAs from an SQS queue.
+
+For most chains, the VAA includes a `txHash` field that identifies the transaction that generated the VAA
+(for the particular case of Solana and Aptos, the value of this field is more like an indirect pointer to the transaction).
+
+The service uses the value of this `txHash` field to query the associated chain's node RPC service, obtaining the transaction's metadata (e.g.: the sender's address).
+
+This data is persisted in MongoDB the `globalTransaction` collection, as in the `originTx` object.
+
+## Retry logic
+
+Sometimes, fetching tx metadata from a node fails, e.g.:
+1. The VAA was emitted with a low consistency level and the tx was not confirmed.
+2. The particular node we're querying is down or we've reached a request limit.
+
+When this happens, the service will retry until both these conditions are met:
+* At least 10 minutes have passed since the VAA was emitted (this mitigates problem 1)
+* At least 4 retries have been made (this mitigates problem 2)
+
+This service processes VAAs in a sequential order, there is no concurrency.
+
+## Backfiller
+
+In the `cmd/backfiller` directory, there is a backfiller program that can be used to:
+* Fill gaps in processing (e.g. due to downtime)
+* Reprocess old records (e.g. overwrite them, this can be useful to apply database model changes)
+
+The backfiller processes VAAs between two input dates.
+
+VAAs will be iterated in a descending timestamp order, that is, newer VAAs are processed first. The rationale behind this is that old VAAs are not as relevant as recent ones.