doc: Add indexation and search usage instructions

README.md

@@ -1,2 +1,25 @@
 # raptor-rs
 Raptor, the new RISE
+
+## Usage
+
+First you need to generate the index files.
+
+```bash
+$ cargo build --release --bin raptor-indexer
+$ time ./target/release/raptor-indexer products.json_lines
+```
+
+Once the command finished indexing you will have 3 files that compose the index:
+- The `xxx.map` represent the fst map.
+- The `xxx.idx` represent the doc indexes matching the words in the map.
+- The `xxx.sst` is a file that contains all the fields and the values asociated with it, it is passed to the internal RocksDB.
+
+Now you can easily use `raptor-search` or `raptor-http` with only the prefix name of the files. (e.g. relaxed-colden).
+
+```bash
+$ cargo run --bin raptor-search -- relaxed-colden
+$ cargo run --bin raptor-http -- relaxed-colden
+```
+
+Note: If you have performance issues run the searcher in release mode (i.e. `--release`).