lnav config for synpase logs (#14953)

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+# `lnav` config for Synapse logs
+
+[lnav](https://lnav.org/) is a log-viewing tool. It is particularly useful when 
+you need to interleave multiple log files, or for exploring a large log file
+with regex filters. The downside is that it is not as ubiquitous as tools like
+`less`, `grep`, etc.
+
+This directory contains an `lnav` [log format definition](
+    https://docs.lnav.org/en/v0.10.1/formats.html#defining-a-new-format
+) for Synapse logs as
+emitted by Synapse with the default [logging configuration](
+    https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html#log_config
+). It supports lnav 0.10.1 because that's what's packaged by my distribution.
+
+This should allow lnav:
+
+- to interpret timestamps, allowing log interleaving;
+- to interpret log severity levels, allowing colouring by log level(!!!);
+- to interpret request IDs, allowing you to skip through a specific request; and
+- to highlight room, event and user IDs in logs.
+
+See also https://gist.github.com/benje/e2ab750b0a81d11920d83af637d289f7 for a
+ similar example.
+
+## Example
+
+[![asciicast](https://asciinema.org/a/556133.svg)](https://asciinema.org/a/556133)
+
+## Tips
+
+- `lnav -i /path/to/synapse/checkout/contrib/lnav/synapse-log-format.json`
+- `lnav my_synapse_log_file` or `lnav synapse_log_files.*`, etc.
+- `lnav --help` for CLI help.
+
+Within lnav itself:
+
+- `?` for help within lnav itself.
+- `q` to quit.
+- `/` to search a-la `less` and `vim`, then `n` and `N` to continue searching 
+  down and up.
+- Use `o` and `O` to skip through logs based on the request ID (`POST-1234`, or
+  else the value of the [`request_id_header`](
+    https://matrix-org.github.io/synapse/latest/usage/configuration/config_documentation.html?highlight=request_id_header#listeners
+  ) header). This may get confused if the same request ID is repeated among 
+  multiple files or process restarts.
+- ???
+- Profit