[gnome-terminal] docs: Add skeleton README.md with basic steps for building

[Christian Persch <chpe src gnome org>]

commit 51994fdb014e78f5ef2aeac07eed0f71ac30dd31
Author: Christian Persch <chpe src gnome org>
Date:   Fri May 21 23:19:46 2021 +0200

    docs: Add skeleton README.md with basic steps for building
    
    Fixes: https://gitlab.gnome.org/GNOME/gnome-terminal/-/issues/361

 README.md | 118 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
---
diff --git a/README.md b/README.md
new file mode 100644
index 00000000..080988ed
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+++ b/README.md
@@ -0,0 +1,118 @@
+GNOME TErminal
+==============
+
+Source & Releases
+-----------------
+
+To get the source code, use
+```
+$ git clone https://gitlab.gnome.org/GNOME/gnome-terminal
+```
+
+To get the source for a release version, use the corresponding git tag, or
+download a tarball at
+https://gitlab.gnome.org/GNOME/gnome-terminal/-/archive/TAG/gnome-terminal-TAG.tar.bz2
+replacing `TAG` with the desired tag's name (e.g. `3.40.0`). Older releases are also
+available at https://download.gnome.org/sources/gnome-terminal .
+
+Building from source
+--------------------
+
+You will most likely need to also build `vte` from source; see 
https://gitlab.gnome.org/GNOME/vte/-/blob/master/README.md .
+
+Start by installing the build dependencies, and a C++ compiler.
+
+For fedora and related distributions, use
+```
+sudo dnf build-dep vte291 gnome-terminal
+sudo dnf install g++
+```
+while for debian and related distributions, use
+```
+sudo apt-get build-dep libvte-2.91-0 gnome-terminal
+sudo apt-get install g++
+```
+
+First build `vte` according to its own instructions. Then:
+```
+$ # Get the source code
+$ git clone https://gitlab.gnome.org/GNOME/gnome-terminal
+$
+$ # Change to the toplevel directory
+$ cd gnome-terminal
+$
+$ # Run the configure script (choose an appropriate path instead of "/some/where"!)
+$ # Don't forget to make sure that pkg-config can find your self-build vte!
+$ # e.g. by doing:
+$ # export PKG_CONFIG_PATH=/some/where/lib64/pkg-config:$PKG_CONFIG_PATH
+$ meson _build --prefix=/some/where
+$
+$ # Build
+$ ninja -C _build
+$
+$ # Install
+$ ninja -C _build install
+```
+
+* By default, GNOME Terminal will install under `/usr/local`, which is not usually
+the right choice. You can customize the prefix directory by `--prefix` option, e.g.
+if you want to install GNOME-TERMINAL under `~/foobar`, you should run
+`meson _build --prefix=$HOME/foobar`. If you already run the configure script before,
+you should also pass `--reconfigure` option to it.
+
+* You may need to execute `ninja -C _build install` as root
+(i.e. `sudo ninja -C _build install`) if installing to system directories. Use a
+user-writable directory as `--prefix` instead to avoid that.
+
+* You may need to set the gsettings schema path so that gnome-terminal can find
+its schemas. E.g. use
+```
+$ export GSETTINGS_SCHEMA_DIR=/some/where/share/glib-2.0/schemas
+```
+and if you skipped the `ninja install` step, you need to create the
+gsettings schema cache yourself.
+
+* Since GNOME Terminal uses a D-Bus activated server, you cannot simply run
+the self-built gnome-terminal directly. Instead, you need to start the new `gnome-terminal-server` directly 
using
+```
+$ ./_build/src/gnome-terminal-server --app-id test.Terminal &
+```
+and then you have 10s time to open a window in that server using
+```
+$ ./_build/src/gnome-terminal --app-id test.Terminal
+```
+
+Also see https://wiki.gnome.org/Apps/Terminal/Debugging for more information.
+
+Debugging
+---------
+
+After installing GNOME-TERMINAL with `-Ddebugg=true` flag, you can use `GNOME-TERMINAL_DEBUG` variable to 
control
+GNOME-TERMINAL to print out the debug information
+
+```
+# You should change gnome-terminal-[2.91] to the version you build
+$ GNOME-TERMINAL_DEBUG=selection ./_build/src/app/gnome-terminal-2.91
+
+# Or, you can mixup with multiple logging level
+$ GNOME-TERMINAL_DEBUG=selection,draw,cell ./_build/src/app/gnome-terminal-2.91
+
+$ Or, you can use `all` to print out all logging message
+$ GNOME-TERMINAL_DEBUG=all ./_build/src/app/gnome-terminal-2.91
+```
+
+For logging level information, please refer to enum [Gnome-TerminalDebugFlags](src/debug.h).
+
+Contributing
+------------
+
+Bugs should be filed here: https://gitlab.gnome.org/GNOME/gnome-terminal/issues/
+Please note that this is a bug tracker to be used for developers of GNOME Terminal,
+and contributors of code, documentation, and translations to GNOME Terminal,
+and *not a support forum*.
+
+If you are an end user, always file bugs in your distribution's bug tracker, or use their
+support forums.
+
+If you want to provide a patch, please attach them to an issue in GNOME
+GitLab, in the format output by the `git format-patch` command.