Add readme

README.md

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+## Description
+
+Fin Spell is an experimental Finnish spellchecking add-on for Firefox. It tries
+to closely emulate the built-in spellchecking feature. This project was started,
+because the old Finnish spellchecking add-on, Mozvoikko, is a XUL add-on and
+they were deprecated, and Hunspell or MySpell formatted dictionaries aren't well
+suited for Finnish.
+
+This add-on is based on libvoikko, like the old Mozvoikko.
+
+## Limitations
+
+Users can't add new words, the add-on must be upgraded to use a new
+dictionary.
+
+## Compiling libvoikko
+
+I did this on Ubuntu 19.04. You can copy these commands all at once to the
+terminal.
+
+```
+# Get emsdk, I couldn't compile with the latest version.
+git clone https://github.com/juj/emsdk.git
+cd emsdk
+./emsdk update-tags
+./emsdk install sdk-1.38.30-64bit
+./emsdk activate sdk-1.38.30-64bit
+source ./emsdk_env.sh
+cd ..
+
+# Libvoikko. I used version rel-libvoikko-4.2
+git clone https://github.com/voikko/corevoikko
+cd corevoikko/libvoikko
+git checkout rel-libvoikko-4.2 
+
+# Get the dictionary.
+# sha256sum: eb3730ec8c3ff8be444479540ce33566d98cc19ecdda1077427556d4e48ad431
+wget https://www.puimula.org/htp/testing/voikko-snapshot-v5/dict-morpho.zip
+unzip dict-morpho.zip
+rm dict-morpho.zip
+
+# Needs automake, autocong and pkg-config packages.
+# Install libtool if you get error: possibly undefined macro: AC_LIBTOOL_WIN32_DLL.
+# You might need to install libtinfo5 and openjdk-X-jdk also.
+./autogen.sh
+js/configure.sh
+js/build.sh preload
+# libvoikko.data, libvoikko_api.js, libvoikko.js and libvoikko.wasm which
+# are used in this add-on are in js/.
+```
+
+## Other requirements
+
+content_scripts/jquery_highlight_combined.js is a concatenation of jQuery and
+highlight-within-textarea JavaScript files.
+
+https://code.jquery.com/jquery-3.3.1.min.js
+
+https://github.com/lonekorean/highlight-within-textarea/raw/master/jquery.highlight-within-textarea.js
+
+content_scripts/jquery.highlight-within-textarea.css's source is
+https://github.com/lonekorean/highlight-within-textarea/raw/master/jquery.highlight-within-textarea.css.
+
+## License
+
+GPL3.