Add stub README files for components

sf-stream/README.md

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+# sf-stream

sf-utils/README.md

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+# sf-utils

sf-writer/README.md

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+# sf-writer
+
+## Data request ranges
+
+Data request ranges are composed of:
+
+- start_pulse_id (first pulse_id to be included in the file)
+- stop_pulse_id (last pulse_id to be included in the file)
+- pulse_id_step (how many pulses to skip between images.)
+
+pulse_id_step can be used to write data at different frequencies:
+
+- pulse_id_step == 1 (100Hz, write very pulse_id)
+- pulse_id_step == 2 (50hz, write every second pulse)
+- pulse_id_step == 10 (10Hz, write every 10th pulse)
+
+The next pulse_id to be written is calculated internally as:
+
+```c++
+auto next_pulse_id = currnet_pulse_id + pulse_id_step;
+```
+
+The loop criteria for writing is:
+
+```c++
+for (
+        auto curr_pulse_id = start_pulse_id; 
+        curr_pulse_id <= stop_pulse_id;
+        curr_pulse_id += pulse_id_step
+) {
+    // Write curr_pulse_id to output file.
+}
+```
+
+**Warning**
+
+If your stop_pulse_id cannot be reached by adding step_pulse_id to 
+start_pulse_id (start_pulse_id + (n * pulse_id_step) != stop_pulse_id for any n)
+it will not be included in the final file.