Add README to manuscript (#134)

Add a README.md to manuscript with descriptions of the files
inside manuscript folder and instructions on how to build the PDF
through Makefile.

Author: santisoler

manuscript/README.md

@@ -0,0 +1,61 @@
+# LaTeX sources for the manuscript
+
+This folder contains all the files needed to build two PDF versions of the
+manuscript using LaTeX: one for the preprint and another one for submitting
+*Geophysical Journal International*.
+The two main LaTeX documents are:
+
+- `preprint.tex`: for building the preprint
+- `gji.tex`: for building the manuscript using the GJI template
+
+These two files only contain a header and some configurations, but the content
+of the article is split in separate files, which are then imported directly
+into `preprint.tex` and `gji.tex`.
+The extra files are:
+
+- `abstract.tex`: contains the text for the abstract
+- `content.tex`: contains the body of the paper, with sections, figures,
+    equations, etc.
+- `appendix.tex`: contains the appendix.
+- `variables.tex`: defines some variables that contain additional information
+    like the title of the manuscript, information about the authors, keywords
+    and more.
+
+The information about the references can be found inside `references.bib`,
+while the bibliography styles used for the preprint and the submission
+manuscript are inside `apalike-doi.bst` and `gji.bst`, respectively.
+
+This folder also contains the files provided by GJI to build the manuscript
+using their template (`gji.cls`, `gji_extra.sty` and `times.sty`).
+
+The figures of the article can be found under the `figs` directory.
+They are automatically created by the Jupyter notebooks inside the `notebooks`
+folder.
+These notebooks also store some numerical values that we include in the
+manuscript as LaTeX variables defined in files under the `results` folder.
+They are also automatically created by the notebooks.
+
+## How to build the manuscript
+
+In order to build the manuscript, you need to install a LaTeX distribution and
+the `latexmk` tool.
+
+The `Makefile` has rules for building the PDF of the manuscript.
+
+To build the preprint PDF, run:
+
+```
+make preprint
+```
+
+To build the PDF of the submission manuscript, run:
+
+```
+make gji
+```
+
+You can also ask for a word and figures count using:
+
+```
+make word-count
+```