docs: document environment variables

fixes #5828

License: MIT
Signed-off-by: Steven Allen <steven@stebalien.com>

docs/environment-variables.md

+# go-ipfs environment variables
+
+## `LIBP2P_TCP_REUSEPORT` (`IPFS_REUSEPORT`)
+
+go-ipfs tries to reuse the same source port for all connections to improve NAT
+traversal. If this is an issue, you can disable it by setting
+`LIBP2P_TCP_REUSEPORT` to false.
+
+This variable was previously `IPFS_REUSEPORT`.
+
+Default: true
+
+## `IPFS_PATH`
+
+Sets the location of the IPFS repo (where the config, blocks, etc.
+are stored).
+
+Default: ~/.ipfs
+
+## `IPFS_LOGGING`
+
+Sets the log level for go-ipfs. It can be set to one of:
+
+* `CRITICAL`
+* `ERROR`
+* `WARNING`
+* `NOTICE`
+* `INFO`
+* `DEBUG`
+
+Logging can also be configured (on a subsystem by subsystem basis) at runtime
+with the `ipfs log` command.
+
+Default: `ERROR`
+
+## `IPFS_LOGGING_FMT`
+
+Sets the log message format. Can be one of:
+
+* `color`
+* `nocolor`
+
+Default: `color`
+
+## `GOLOG_FILE`
+
+Sets the file to which go-ipfs logs. By default, go-ipfs logs to standard error.
+
+## `GOLOG_TRACING_FILE`
+
+Sets the file to which go-ipfs sends tracing events. By default, tracing is
+disabled.
+
+This log can be read at runtime (without writing it to a file) using the `ipfs
+log tail` command.
+
+Warning: Enabling tracing will likely affect performance.
+
+## `IPFS_FUSE_DEBUG`
+
+Enables fuse debug logging.
+
+Default: false
+
+## `YAMUX_DEBUG`
+
+Enables debug logging for the yamux stream muxer.
+
+Default: false
+
+## `IPFS_FD_MAX`
+
+Sets the file descriptor limit for go-ipfs. If go-ipfs fails to set the file
+descriptor limit, it will log an error.
+
+Defaults: 2048
+
+## `IPFS_DIST_PATH`
+
+URL from which go-ipfs fetches repo migrations (when the daemon is launched with
+the `--migrate` flag).
+
+Default: https://ipfs.io/ipfs/$something (depends on the IPFS version)
+
+## `LIBP2P_MUX_PREFS`
+
+Tells go-ipfs which multiplexers to use in which order.
+
+Default: "/yamux/1.0.0 /mplex/6.7.0"