docs: add functionality, and usage

others:

* remove drop command from sql migration

docs/filters.md

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+## Writing filters
+
+Filters must conform to the interface:
+
+```go
+type Filter interface {
+	Execute(ctx context.Context, inputTransaction fetch.Transaction) (next bool, err error)
+}
+```
+
+See examples in the `internal/filter` folder.

docs/functionality.md

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+## Functionality
+
+### Head syncer
+
+Opens a websocket connection and processes live transactions.
+
+### Janitor
+
+Periodically checks for missed (and historical) blocks missed by the head syncer and queues them for processing. A gap range is processed twice to guarantee there is no missing block.
+
+### Pipeline
+
+Fetches a block and executes the filters in serial order for every transaction in the block before finally committing the block to the store.
+
+### Filter
+
+Processes a transaction and passes it on to the next filter or terminates the pipeline for that transaction if it is irrelevant.
+
+### Store schema
+
+- The `blocks` table keeps track of processed blocks.
+- The `syncer_meta` table keeps track of the lower_bound cursor. Below the lower_bound cursor, all blocks are guarnteed to have been processsed hence it is safe to trim the `blocks` table below that pointer.
+
+### GraphQL
+
+- Fetches a block (and some of its header details), transactions and transaction receipts embedded within the transaction object in a single call.
+
+## Caveats
+
+- Blocks are not guaranteed to be processed in order, however a low concurrency setting would somewhat give an "in-order" behaviour (not to be relied upon in any case).

docs/usage.md

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+## Requirements
+
+- Celo (geth) node with GraphQL enabled
+- Postgres 14+
+
+## Running
+
+### 1. Run migrations
+
+Run the migrations inside the `migrations` folder.
+
+### 2. Update the config
+
+The base config is described in `config.toml`. Values can be overriden with env variables e.g. to disable metrics, set `METRICS_GO_PROCESS=false`.
+
+### 3. Start the service:
+
+**Compiling**:
+
+- Requires CGO_ENABLED=1
+- Prebuilt binaries (for amd64 only) available on the releases page
+
+**Docker**:
+
+- `docker pull ghcr.io/grassrootseconomics/cic-chain-events/cic-chain-events:latest`
+
+After compiling or within a Docker container:
+
+`$ ./cic-chain-events`
+
+Optional flags:
+
+- `-config` - `config.toml` file path
+- `-debug` - Enable/disable debug level logs
+- `-queries` - `queries.sql` file path
+