[libgdata: 1/3] docs: Port README to Markdown

[Philip Withnall <pwithnall src gnome org>]

commit d4f5b87507c43d6417c0af34b0db6615043caadb
Author: Philip Withnall <pwithnall endlessos org>
Date:   Mon Jul 12 13:59:34 2021 +0100

    docs: Port README to Markdown
    
    Signed-off-by: Philip Withnall <pwithnall endlessos org>

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+# libgdata
+
+libgdata is a GLib-based library for accessing online service APIs using the
+GData protocol — most notably, Google's services. It provides APIs to access
+the common Google services, and has full asynchronous support.
+
+See the test programs in `gdata/tests/` for simple examples of how to use the
+code.
+
+libgdata is API and ABI stable.
+
+## Dependencies
+
+ * glib-2.0 ≥ 2.44.0
+ * libxml-2.0
+ * gio-2.0 ≥ 2.44.0
+ * libsoup-2.4 ≥ 2.42.0
+ * liboauth ≥ 0.9.4
+ * json-glib ≥ 0.15.0
+
+If compiling with `--enable-gnome` (for GNOME support):
+ * gcr-base-3
+ * goa-1.0 ≥ 3.8
+
+If compiling the demos:
+ * gtk+-3.0 ≥ 2.91.2
+
+## Environment variables
+
+If the environment variable `LIBGDATA_DEBUG` is set to one of the following
+values, libgdata will give debug output (at various levels):
+ * `0`: Output no debug messages or network logs
+ * `1`: Output debug messages, but not network logs
+ * `2`: Output debug messages and network traffic headers
+ * `3`: Output debug messages and full network traffic logs, redacting usernames,
+   passwords and auth. tokens
+ * `4`: Output debug messages and full network traffic logs, and don't redact
+   usernames, passwords and auth. tokens
+
+If `LIBGDATA_DEBUG` is unset, no debug output will be produced.
+
+So, to debug a program which uses libgdata, run it from a terminal with the
+following command:
+
+```
+LIBGDATA_DEBUG=3 ./my-program-name &> libgdata.log
+```
+
+## Deprecation guards
+
+If `LIBGDATA_DISABLE_DEPRECATED` is defined when compiling against libgdata, all
+deprecated API will be removed from included headers.
+
+## Licensing
+
+libgdata is licensed under the LGPL; see COPYING for more details.
+
+## Contact
+
+Philip Withnall <philip tecnocode co uk>
+https://wiki.gnome.org/Projects/libgdata