[gjs: 1/2] Documentation: add documentation for ENV variables

[Philip Chimento <pchimento src gnome org>]

commit 9714bad0d3ba1864336776d7738ea59be54790d9
Author: Andy Holmes <andrew g r holmes gmail com>
Date:   Sat Aug 15 12:50:37 2020 -0700

    Documentation: add documentation for ENV variables

 doc/Environment.md | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 104 insertions(+)
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+## Environment
+
+GJS allows runtime configuration with a number of environment variables.
+
+### General
+
+* `GJS_PATH`
+  
+  Set this variable to a list of colon-separated (`:`) paths (just like `PATH`),
+  to add them to the search path for the importer. Use of the `--include-path`
+  command-line option is preferred over this variable.
+
+* `GJS_ABORT_ON_OOM`
+  
+  > NOTE: This feature is not well tested.
+  
+  Setting this variable to any value causes GJS to exit when an out-of-memory
+  condition is encountered, instead of just printing a warning.
+  
+### JavaScript Engine
+
+* `JS_GC_ZEAL`
+  
+  Enable GC zeal, a testing and debugging feature that helps find GC-related
+  bugs in JSAPI applications. See the [Hacking][hacking-gczeal] and the
+  [JSAPI Documentation][mdn-gczeal] for more information about this variable.
+
+* `GJS_DISABLE_JIT`
+  
+  Setting this variable to any value will disable JIT compiling in the
+  JavaScript engine.
+
+
+### Debugging
+  
+* `GJS_DEBUG_HEAP_OUTPUT`
+
+  In addition to `System.dumpHeap()`, you can dump a heap from a running program
+  by starting it with this environment variable set to a path and sending it the
+  `SIGUSR1` signal.
+
+* `GJS_DEBUG_OUTPUT`
+  
+  Set this to "stderr" to log to `stderr` or a file path to save to.
+
+* `GJS_DEBUG_TOPICS`
+  
+  Set this to a semi-colon delimited (`;`) list of prefixes to allow to be
+  logged. Prefixes include:
+  
+   * "JS GI USE"
+   * "JS MEMORY"
+   * "JS CTX"
+   * "JS IMPORT"
+   * "JS NATIVE"
+   * "JS KP ALV"
+   * "JS G REPO"
+   * "JS G NS"
+   * "JS G OBJ"
+   * "JS G FUNC"
+   * "JS G FNDMTL"
+   * "JS G CLSR"
+   * "JS G BXD"
+   * "JS G ENUM"
+   * "JS G PRM"
+   * "JS G ERR"
+   * "JS G IFACE"
+
+* `GJS_DEBUG_THREAD`
+
+  Set this variable to print the thread number when logging.
+
+* `GJS_DEBUG_TIMESTAMP`
+
+  Set this variable to print a timestamp when logging.
+
+
+### Testing
+
+* `GJS_COVERAGE_OUTPUT`
+  
+  Set this variable to define an output path for code coverage information. Use
+  of the `--coverage-output` command-line option is preferred over this
+  variable.
+
+* `GJS_COVERAGE_PREFIXES`
+  
+  Set this variable to define a colon-separated (`:`) list of prefixes to output
+  code coverage information for. Use of the `--coverage-prefix` command-line
+  option is preferred over this variable.
+
+* `GJS_ENABLE_PROFILER`
+  
+  Set this variable to `1` to enable or `0` to disable the profiler. Use of the
+  `--profile` command-line option is preferred over this variable.
+
+* `GJS_TRACE_FD`
+
+  The GJS profiler is integrated directly into Sysprof via this variable. It not
+  typically useful to set this manually.
+
+
+[hacking-gczeal]: https://gitlab.gnome.org/GNOME/gjs/blob/master/doc/Hacking.md#gc-zeal
+[mdn-gczeal]: https://developer.mozilla.org/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/JS_SetGCZeal